Manufacturing method for selenium preloaded mesoporous carbon cathode for alkali metal-selenium secondary battery

ABSTRACT

A method of producing a pre-selenized (selenium-preloaded) active cathode layer for a rechargeable alkali metal-selenium cell; the method comprising: (a) preparing an integral layer of mesoporous structure having pore sizes from 0.5 nm to 50 nm (preferably from 0.5 nm to 5 nm) and a specific surface area from 100 to 3,200 m 2 /g; (b) preparing an electrolyte comprising a solvent and a selenium source; (c) preparing an anode; and (d) bringing the integral layer and the anode in ionic contact with the electrolyte and imposing an electric current between the anode and the integral layer (serving as a cathode) to electrochemically deposit nanoscaled selenium particles or coating on the graphene surfaces. The selenium particles or coating have a thickness or diameter smaller than 20 nm (preferably &lt;10 nm, more preferably &lt;5 nm or even &lt;3 nm) and preferably occupy a weight fraction of at least 70% (preferably &gt;90% or even &gt;95%).

FIELD OF THE INVENTION

The present invention provides a unique cathode composition andstructure in a secondary or rechargeable alkali metal-selenium battery,including the lithium-selenium battery and sodium-selenium battery, anda process for producing same.

BACKGROUND

Rechargeable lithium-ion (Li-ion) and lithium metal batteries (includingLi-sulfur and Li metal-air batteries) are considered promising powersources for electric vehicle (EV), hybrid electric vehicle (HEV), andportable electronic devices, such as lap-top computers and mobilephones. Lithium as a metal element has the highest capacity (3,861mAh/g) compared to any other metal or metal-intercalated compound as ananode active material (except Li_(4.4)Si, which has a specific capacityof 4,200 mAh/g). Hence, in general, Li metal batteries have asignificantly higher energy density than lithium ion batteries.

Historically, rechargeable lithium metal batteries were produced usingnon-lithiated compounds having relatively high specific capacities, suchas TiS₂, MoS₂, MnO₂, CoO₂, and V₂O₅, as the cathode active materials,which were coupled with a lithium metal anode. When the battery wasdischarged, lithium ions were transferred from the lithium metal anodethrough the electrolyte to the cathode, and the cathode becamelithiated. Unfortunately, upon repeated charges/discharges, the lithiummetal resulted in the formation of dendrites at the anode thatultimately grew to penetrate through the separator, causing internalshorting and explosion. As a result of a series of accidents associatedwith this problem, the production of these types of secondary batterieswas stopped in the early 1990's, giving ways to lithium-ion batteries.

In lithium-ion batteries, pure lithium metal sheet or film was replacedby carbonaceous materials as the anode. The carbonaceous materialabsorbs lithium (through intercalation of lithium ions or atoms betweengraphene planes, for instance) and desorbs lithium ions during there-charge and discharge phases, respectively, of the lithium ion batteryoperation. The carbonaceous material may comprise primarily graphitethat can be intercalated with lithium and the resulting graphiteintercalation compound may be expressed as Li_(x)C₆, where x istypically less than 1.

Although lithium-ion (Li-ion) batteries are promising energy storagedevices for electric drive vehicles, state-of-the-art Li-ion batterieshave yet to meet the cost and performance targets. Li-ion cellstypically use a lithium transition-metal oxide or phosphate as apositive electrode (cathode) that de/re-intercalates Li⁺ at a highpotential with respect to the carbon negative electrode (anode). Thespecific capacity of lithium transition-metal oxide or phosphate basedcathode active material is typically in the range of 140-180 mAh/g. As aresult, the specific energy of commercially available Li-ion cells istypically in the range of 120-240 Wh/kg, most. These specific energyvalues are two to three times lower than what would be required forbattery-powered electric vehicles to be widely accepted.

With the rapid development of hybrid (HEV), plug-in hybrid electricvehicles (HEV), and all-battery electric vehicles (EV), there is anurgent need for anode and cathode materials that provide a rechargeablebattery with a significantly higher specific energy, higher energydensity, higher rate capability, long cycle life, and safety. Two of themost promising energy storage devices are the lithium-sulfur (Li—S) celland lithium-selenium (Li—Se) cell since the theoretical capacity of Liis 3,861 mAh/g, that of S is 1,675 mAh/g, and that of Se is 675 mAh/g.Compared with conventional intercalation-based Li-ion batteries, Li—Sand Li—Se cells have the opportunity to provide a significantly higherenergy density (a product of capacity and voltage). With a significantlyhigher electronic conductivity, Se is a more effective cathode activematerial and, as such, Li—Se potentially can exhibit a higher ratecapability.

However, Li—Se cell is plagued with several major technical problemsthat have hindered its widespread commercialization:

-   (1) All prior art Li—Se cells have dendrite formation and related    internal shorting issues;-   (2) The cell tends to exhibit significant capacity decay during    discharge-charge cycling. This is mainly due to the high solubility    of selenium and lithium poly selenide anions formed as reaction    intermediates during both discharge and charge processes in the    polar organic solvents used in electrolytes. During cycling, the    anions can migrate through the separator to the Li negative    electrode whereupon they are reduced to solid precipitates, causing    active mass loss. In addition, the solid product that precipitates    on the surface of the positive electrode during discharge becomes    electrochemically irreversible, which also contributes to active    mass loss. This phenomenon is commonly referred to as the Shuttle    Effect. This process leads to several problems: high self-discharge    rates, loss of cathode capacity, corrosion of current collectors and    electrical leads leading to loss of electrical contact to active    cell components, fouling of the anode surface giving rise to    malfunction of the anode, and clogging of the pores in the cell    membrane separator which leads to loss of ion transport and large    increases in internal resistance in the cell.-   (3) Presumably, nanostructured mesoporous carbon materials could be    used to hold the Se or lithium polyselenide in their pores,    preventing large out-flux of these species from the porous carbon    structure through the electrolyte into the anode. However, the    fabrication of the proposed highly ordered mesoporous carbon    structure requires a tedious and expensive template-assisted    process. It is also challenging to load a large proportion of    selenium into these mesoscaled pores using a physical vapor    deposition or solution precipitation process. Typically the maximum    loading of Se in these porous carbon structures is less than 50%.

Despite the various approaches proposed for the fabrication of highenergy density Li—Se cells, there remains a need for cathode materials,production processes, and cell operation methods that retard theout-diffusion of Se or lithium polyselenide from the cathodecompartments into other components in these cells, improve theutilization of electroactive cathode materials (Se utilizationefficiency), and provide rechargeable Li—Se cells with high capacitiesover a large number of cycles.

Most significantly, lithium metal (including pure lithium, lithiumalloys of high lithium content with other metal elements, orlithium-containing compounds with a high lithium content; e.g. >80% orpreferably >90% by weight Li) still provides the highest anode specificcapacity as compared to essentially all other anode active materials(except pure silicon, but silicon has pulverization issues). Lithiummetal would be an ideal anode material in a lithium-selenium secondarybattery if dendrite related issues could be addressed.

Sodium metal (Na) and potassium metal (K) have similar chemicalcharacteristics to Li and the selenium cathode in sodium-selenium cells(Na—Se batteries) or potassium-selenium cells (K—Se) face the sameissues observed in Li—S batteries, such as: (i) low active materialutilization rate, (ii) poor cycle life, and (iii) low Coulumbicefficiency. Again, these drawbacks arise mainly from insulating natureof Se, dissolution of polyselenide intermediates in liquid electrolytes(and related Shuttle effect), and large volume change duringcharge/discharge.

Hence, an object of the present invention is to provide a rechargeableLi—Se battery that exhibits an exceptionally high specific energy orhigh energy density. One particular technical goal of the presentinvention is to provide a Li metal-selenium or Li ion-selenium cell witha cell specific energy greater than 300 Wh/Kg, preferably greater than350 Wh/Kg, and more preferably greater than 400 Wh/Kg (all based on thetotal cell weight).

It may be noted that in most of the open literature reports (scientificpapers) and patent documents, scientists or inventors choose to expressthe cathode specific capacity based on the selenium or lithiumpolyselenide weight alone (not the total cathode composite weight), butunfortunately a large proportion of non-active materials (those notcapable of storing lithium, such as conductive additive and binder) istypically used in their Li—Se cells. For practical use purposes, it ismore meaningful to use the cathode composite weight-based capacityvalue.

A specific object of the present invention is to provide a rechargeablelithium-selenium cell based on rational materials and battery designsthat overcome or significantly reduce the following issues commonlyassociated with conventional Li—Se cells: (a) dendrite formation(internal shorting); (b) low electric and ionic conductivities ofselenium, requiring large proportion (typically 30-55%) of non-activeconductive fillers and having significant proportion of non-accessibleor non-reachable selenium or lithium polyselenide); (c) dissolution oflithium polyselenide in electrolyte and migration of dissolved lithiumpolyselenide from the cathode to the anode (which irreversibly reactwith lithium at the anode), resulting in active material loss andcapacity decay (the shuttle effect); and (d) short cycle life.

In addition to overcoming the aforementioned problems, another object ofthe present invention is to provide a simple, cost-effective, andeasy-to-implement approach to preventing potential Li metaldendrite-induced internal short circuit and thermal runaway problems inLi metal-selenide batteries.

SUMMARY OF THE INVENTION

The present invention provides a unique cathode composition andstructure in a secondary or rechargeable alkali metal-selenium battery,including the lithium-selenium battery, sodium-selenium battery, andpotassium-selenium battery. The lithium-selenium battery can include thelithium metal-selenium battery (having lithium metal as the anode activematerial and selenium as the cathode active material) and the lithiumion-selenium battery (e.g. Si or graphite as the anode active materialand selenium as the cathode active material). The sodium-seleniumbattery can include the sodium metal-selenium battery (having sodiummetal as the anode active material and selenium as the cathode activematerial) and the sodium ion-selenium battery (e.g. hard carbon as theanode active material and selenium as the cathode active material).

The present invention provides an electrochemical method of producing apre-selenized active cathode layer for use in a rechargeable alkalimetal-selenium cell. The term “pre-selenized” means pre-loading seleniuminto a cathode active material before this cathode active material isincorporated into a battery cell (if this pre-selenization procedure isconducted outside of the intended lithium-selenium cell) or before thebattery cell is operated to provide power to an external device (if thispre-selenization procedure is conducted in situ inside the intendedlithium-selenium cell during the first charge cycle).

Such an electrochemical method is surprisingly capable of uniformlydepositing an ultra-thin selenium (Se) coating layer or ultra-smallsmall Se particles (<20 nm, more preferably and typically <10 nm, mosttypically and preferably <5 nm, or even <3 nm) on massive pore wallsurfaces or inside the pores of a mesoporous structure, yet achieving alarge proportion of Se (the cathode active material) relative to thesupporting mesoporous structure. These electrochemically deposited Secoating or particles remain well-adhered to the pore wall surfaces orlodged in the mesoscaled pores during repeated charges/discharges,enabling an unusually high long cycle life. The ultra-thin dimensionsalso enable high storing/releasing rates of alkali metal ions (Li⁺, Na⁺,and/or K⁺) and, hence, exceptional rate capability or power density.

For the purpose of describing the preferred embodiments of the instantinvention, Li ions, Li metal, and Li—Se cells are used as examples. But,the same or similar procedures are applicable to other alkali metals andalkali metal-selenium cells (e.g. Na—Se cells and K—Se cells) Thismethod comprises the following four elements, (a)-(d):

-   -   a) Preparing an integral layer of mesoporous graphene structure        having mesoscaled pores (0.5-50 nm in size) and massive surfaces        with a specific surface area greater than 100 m²/g. The porous        structure has a specific surface area preferably >500 m²/g and        more preferably >750 m²/g, and most preferably >1,000 m²/g.

The integral layer of a mesoporous structure comprises primarily acarbon, graphite, metal, or conductive polymer selected from chemicallyetched or expanded soft carbon, chemically etched or expanded hardcarbon, exfoliated activated carbon, chemically etched or expandedcarbon black, chemically etched multi-walled carbon nanotube,nitrogen-doped carbon nanotube, boron-doped carbon nanotube, chemicallydoped carbon nanotube, ion-implanted carbon nanotube, chemically treatedmulti-walled carbon nanotube with an inter-planar separation no lessthan 0.4 nm, chemically expanded carbon nanofiber, chemically activatedcarbon nanotube, chemically treated carbon fiber, chemically activatedgraphite fiber, chemically activated carbonized polymer fiber,chemically treated coke, activated mesophase carbon, mesoporous carbon,electrospun conductive nanofiber, highly separated vapor-grown carbon orgraphite nanofibers (to ensure a specific surface area >100 m²/g evenafter these fibers are packed together), carbon or graphite whisker,carbon nanotube, carbon nanowire, metal nanowire, metal-coated nanowireor nanofiber, conductive polymer-coated nanowire or nanofiber, or acombination thereof.

It may be noted that these carbon/graphite materials, without anychemical etching/expansion treatment, typically contain some graphiticcrystals or graphene domains therein having a spacing between twohexagonal carbon planes or inter-graphene spacing from 0.335 nm to 0.37nm. These materials may be subjected to chemical etching, intercalation,oxidation, fluorination, etc. to expand the spacing from <0.37 nm tobecome greater than 0.5 nm, preferably greater than 0.6 nm, furtherpreferably greater than 1.0 nm, and most desirably from 0.5 nm to 5 nm.Such treatments open up the internal graphitic structure, enabling entryof liquid electrolyte into the pores and making it possible for seleniumor metal selenide species to reside in these pores or get adhered to thepore walls during the subsequent electrochemical deposition (describedbelow in steps (b)-(d)).

These materials, in a particulate or fibrous form, along with anoptional binder material (0-10% by weight), are combined to form amesoporous structure that must still have a specific surface areagreater than 100 m²/g (preferably >500 m²/g and more preferably >750m²/g, and most preferably >1,000 m²/g).

The layer of mesoporous graphene structure may optionally contain 0-49%(preferably 0-30%, more preferably 0-20%, and further preferably 0-10%)by weight of selenium or selenium-containing compound pre-loaded thereinprior to the current electrochemical deposition), based on the totalweights of all ingredients in the layer. Although not preferred, one canpre-load 0.01% to 49% of Se in the mesoporous structure.

-   -   b) Preparing an electrolyte comprising a solvent (preferably        non-aqueous solvent such as organic solvent and/or ionic liquid)        and a selenium source dissolved or dispersed in the solvent;    -   c) Preparing an anode (this anode layer can be an anode active        material layer in an intended alkali metal-selenium cell or an        electrode in an external chamber/reactor that is external or        unrelated to the intended alkali metal-selenium cell); and    -   d) Bringing the integral layer of mesoporous structure and the        anode in ionic contact with the electrolyte (e.g. immersing all        these components in a chamber or reactor being external to the        intended alkali metal-selenium cell, or encasing these three        components inside the battery cell) and imposing an electric        current between the anode and the integral layer of mesoporous        structure (serving as a cathode) with a sufficient current        density for a sufficient period of time to electrochemically        deposit nanoscaled particles or coating of selenium or metal        selenide (e.g. Li₂Se_(x), 2<x<10) in the mesoscaled pores to        form the pre-selenized active cathode layer, wherein the        particles or coating have a thickness or diameter smaller than        20 nm (preferably <10 nm, more preferably <5 nm, and further        preferably <3 nm) and wherein the nanoscaled selenium or metal        selenide particles or coating occupy a weight fraction of        preferably at least 70% (more preferably >80%, even more        preferably >90%, and most preferably >95%) based on the total        weights of the selenium or metal selenide particles or coating        and the mesoporous structure combined.        When these three components (the mesoporous structure, anode,        and electrolyte) are encased inside the intended alkali        metal-selenium cell, nanoselenium or metal selenide is        electrochemically deposited in situ in the cathode inside the        battery cell. When the three components are implemented in an        external container (chamber or reactor outside of the intended        battery cell), nanoselenium or metal selenide is deposited in        the mesoscaled pores through the “external electrochemical        deposition” route.

In one preferred embodiment, the selenium source is selected fromM_(x)Se_(y), wherein x is an integer from 1 to 3 and y is an integerfrom 1 to 10, and M is a metal element selected from an alkali metal, analkaline metal selected from Mg or Ca, a transition metal, a metal fromgroups 13 to 17 of the periodic table, or a combination thereof. In adesired embodiment, the metal element M is selected from Li, Na, K, Mg,Zn, Cu, Ti, Ni, Co, Fe, or Al. In a particularly desired embodiment,M_(x)Se_(y) is selected from Li₂Se₆, Li₂Se₇, Li₂Se₈, Li₂Se₉, Li₂Se₁₀,Na₂Se₆, Na₂Se₇, Na₂Se₈, Na₂Se₉, Na₂Se₁₀, K₂Se₆, K₂Se₇, K₂Se₈, K₂Se₉,K₂Se₁₀, or a combination thereof.

In one embodiment, the anode comprises an anode active material selectedfrom an alkali metal, an alkaline metal, a transition metal, a metalfrom groups 13 to 17 of the periodic table, or a combination thereof.

In one embodiment, the method further comprises a procedure ofdepositing an element Z to the mesoporous structure wherein element Z ismixed with selenium or formed as discrete Z coating or particles havinga dimension less than 100 nm (preferably <20 nm, further preferably <10nm, even more preferably <5 nm, and most preferably <3 nm) and Z elementis selected from Sn, Sb, Bi, S, and/or Te. The procedure of depositingelement Z may be preferably selected from electrochemical deposition,chemical deposition, or solution deposition. We have discovered that theaddition of some amount (less than 50%, preferably less than 20% byweight) of Sn, Sb, Bi, S, or Te can lead to improved cathodeconductivity and/or higher specific capacity.

The electrolyte may further comprise a metal salt selected from lithiumperchlorate (LiClO₄), lithium hexafluorophosphate (LiPF₆), lithiumborofluoride (LiBF₄), lithium hexafluoroarsenide (LiAsF₆), lithiumtrifluoro-metasulfonate (LiCF₃SO₃), bis-trifluoromethyl sulfonylimidelithium (LiN(CF₃SO₂)₂), lithium bis(oxalato)borate (LiBOB), lithiumoxalyldifluoroborate (LiBF₂C₂O₄), lithium oxalyldifluoroborate(LiBF₂C₂O₄), lithium nitrate (LiNO₃), Li-fluoroalkyl-phosphate(LiPF₃(CF₂CF₃)₃), lithium bisperfluoro-ethysulfonylimide (LiBETI),lithium bis(trifluoromethanesulfonyl)imide, lithiumbis(fluorosulfonyl)imide, lithium trifluoromethanesulfonimide (LiTFSI),an ionic liquid salt, sodium perchlorate (NaClO₄), potassium perchlorate(KClO₄), sodium hexafluorophosphate (NaPF₆), potassiumhexafluorophosphate (KPF₆), sodium borofluoride (NaBF₄), potassiumborofluoride (KBF₄), sodium hexafluoroarsenide, potassiumhexafluoroarsenide, sodium trifluoro-metasulfonate (NaCF₃SO₃), potassiumtrifluoro-metasulfonate (KCF₃SO₃), bis-trifluoromethyl sulfonylimidesodium (NaN(CF₃SO₂)₂), sodium trifluoromethanesulfonimide (NaTFSI),bis-trifluoromethyl sulfonylimide potassium (KN(CF₃SO₂)₂), or acombination thereof.

The solvent may be selected from 1,3-dioxolane (DOL),1,2-dimethoxyethane (DME), tetraethylene glycol dimethylether (TEGDME),poly(ethylene glycol) dimethyl ether (PEGDME), diethylene glycol dibutylether (DEGDBE), 2-ethoxyethyl ether (EEE), sulfone, sulfolane, ethylenecarbonate (EC), dimethyl carbonate (DMC), methylethyl carbonate (MEC),diethyl carbonate (DEC), ethyl propionate, methyl propionate, propylenecarbonate (PC), γ-butyrolactone (γ-BL), acetonitrile (AN), ethyl acetate(EA), propyl formate (PF), methyl formate (MF), toluene, xylene, methylacetate (MA), fluoroethylene carbonate (FEC), vinylene carbonate (VC),allyl ethyl carbonate (AEC), a hydrofluoroether, a room temperatureionic liquid solvent, or a combination thereof.

In one preferred embodiment, the electrochemical deposition is conductedbefore the cathode active layer is incorporated into an intendedlithium-selenium (Li—Se) battery cell, Na—Se cell, or K—Se cell. Inother words, the anode, the electrolyte, and the integral layer ofmesoporous structure (serving as a cathode layer) are disposed in anexternal container outside of a lithium-selenium cell. The neededapparatus is similar to an electroplating system. The step ofelectrochemically depositing nanoscaled selenium particles or coating inthe mesoscaled pores is conducted outside the lithium-selenium cell andprior to the battery cell fabrication.

In another embodiment, the anode, the electrolyte, and the integrallayer of porous graphene structure are included inside an alkalimetal-selenium cell (e.g. a lithium-selenium cell). In other words, thebattery cell itself is an electrochemical deposition system forpre-selenization of the cathode and the step of electrochemicallydepositing nanoscaled selenium particles or coating in the mesoscaledpores occurs after the lithium-selenium cell is fabricated. Thiselectrochemical deposition procedure is conducted during the firstcharge cycle of the Li—Se cell.

A special and highly advantageous feature of the inventive method is thenotion that this method enables the selenium or metal selenide to bedeposited in a thin coating or ultra-fine particle form (thus, providingultra-short lithium ion diffusion paths and, hence, ultra-fast reactiontimes for fast battery charges and discharges) while achieving arelatively high proportion of selenium (the active material responsiblefor storing lithium) and, thus, high specific lithium storage capacityof the resulting cathode active layer in terms of high mAh/g (based onthe total weight of the cathode layer, including the masses of theactive material, Se, mesoporous structure, binder resin, and conductivefiller combined). It is of significance to note that one might be ableto use a prior art procedure to deposit small Se particles, but cannotachieve a high Se proportion at the same time, or to achieve a highproportion of Se, but only in large particles or thick film form. Theprior art procedures have not been able to achieve both at the sametime.

This is why it is such an unexpected and highly advantageous thing toachieve a high selenium loading and yet, concurrently, form anultra-thin coating or ultra-small diameter particles of selenium. Thishas not been possible with any prior art selenium loading techniques.For instance, we have been able to deposit nanoscaled selenium particlesor coating that occupy a >90% weight fraction of the cathode layer andyet maintain a coating thickness or particle diameter <3 nm. This isquite a feat in the art of lithium-selenium batteries. In anotherexample, we have achieved a >95% Se loading at an average Se coatingthickness of 4.5-7.1 nm. These ultra-thin dimensions (3-7 nm) enablefacile cathode reactions and nearly perfect selenium utilizationefficiency, something that no prior worker has been able to achieve.

It may be noted that one may begin the electrochemical depositionprocedure, for instance, with a Se source of Li₂Se₉, which may begradually reduced to Li₂Se_(n), where 1<n<9. Thus, one may stop theserial reactions at any point of time to obtain various lithiumpolyselenides. Similarly, one can readily obtain various metal selenides(Na selenides, potassium selenides, etc.).

In certain embodiments, the electrochemical method is conducted in anelectrochemical chamber that is outside of an intended alkalimetal-selenium cell and the method further contains a step of combiningsaid pre-selenized active cathode layer, an alkali metal anode layer,and an electrolyte to form the alkali metal-selenium cell. In certainother embodiments, the method is conducted inside an intended alkalimetal-selenium cell and during the first charge or discharge cycle ofthe cell.

The present invention also provides a pre-selenized active cathode layerproduced by the above-described method and a rechargeable alkalimetal-selenium cell (e.g. lithium-selenium battery cell) that containssuch a cathode layer. In certain embodiments, the pre-selenized cathodefor a rechargeable alkali metal-selenium cell comprises: (A) an integrallayer of a mesoporous structure of a carbon, graphite, metal, orconductive polymer, wherein the mesoporous structure has mesoscaledpores of 0.5-50 nm and a specific surface area greater than 100 m²/g andwherein said carbon, graphite, metal, or conductive polymer is selectedfrom chemically etched or expanded soft carbon, chemically etched orexpanded hard carbon, exfoliated activated carbon, chemically etched orexpanded carbon black, chemically etched multi-walled carbon nanotube,nitrogen-doped carbon nanotube, boron-doped carbon nanotube, chemicallydoped carbon nanotube, ion-implanted carbon nanotube, chemically treatedmulti-walled carbon nanotube with an inter-planar separation no lessthan 0.5 nm, chemically expanded carbon nanofiber, chemically activatedcarbon nanotube, chemically treated carbon fiber, chemically activatedgraphite fiber, chemically activated carbonized polymer fiber,chemically treated coke, activated mesophase carbon, mesoporous carbon,electrospun conductive nanofiber, highly separated vapor-grown carbon orgraphite nanofiber, highly separated carbon nanotube, carbon nanowire,metal nanowire, metal-coated nanowire or nanofiber, conductivepolymer-coated nanowire or nanofiber, or a combination thereof, and (b)nanoparticles or nanocoating of selenium or metal selenide having adiameter or thickness from 0.5 nm to 20 nm, wherein the selenium ormetal selenide resides in the mesoscaled pores and occupies an amountfrom 50% to 99% by weight based on the total weight of the selenium ormetal selenide and the integral layer of mesoporous structure combined.

In certain preferred embodiments, the present invention provides apre-selenized cathode for a rechargeable alkali metal-selenium cell, thecathode comprising (A) an integral layer of a mesoporous structure of acarbon or graphite material, wherein the mesoporous structure hasmesoscaled pores with a pore size of 0.5-5.0 nm and a specific surfacearea from 100 to 3,200 m²/g and wherein the carbon or graphite isselected from chemically etched or expanded soft carbon, chemicallyetched or expanded hard carbon, exfoliated activated carbon, chemicallyetched or expanded carbon black, chemically etched multi-walled carbonnanotube, chemically treated multi-walled carbon nanotube with aninter-planar separation no less than 0.5 nm, chemically expanded carbonnanofiber, chemically treated carbon fiber, chemically activatedgraphite fiber, chemically activated carbonized polymer fiber,chemically treated coke, activated mesophase carbon, mesoporous carbon,electrospun conductive nanofiber, highly separated vapor-grown carbon orgraphite nanofiber, highly separated carbon nanotube, or a combinationthereof, and (b) nanoparticles or nanocoating of selenium or metalselenide having a diameter or thickness from 0.5 nm to 5.0 nm, whereinthe selenium or metal selenide resides in the mesoscaled pores andoccupies an amount from 70% to 95% by weight based on the total weightof the selenium or metal selenide and the integral layer of mesoporousstructure combined.

Preferably, the particles or filaments of thecarbon/graphite/metal/polymer materials in the integral layer ofmesoporous structure are chemically bonded together with an adhesiveresin. Typically, such a rechargeable alkali metal-selenium cellcomprises an anode active material layer, an optional anode currentcollector, a porous separator and/or an electrolyte, a pre-selenizedactive cathode layer, and an optional cathode current collector.

In the invented rechargeable alkali metal-selenium cell, the electrolytemay be selected from polymer electrolyte, polymer gel electrolyte,composite electrolyte, ionic liquid electrolyte, non-aqueous liquidelectrolyte, soft matter phase electrolyte, solid-state electrolyte, ora combination thereof. The electrolyte preferably contains an alkalimetal salt (lithium salt, sodium salt, and/or potassium salt) selectedfrom lithium perchlorate (LiClO₄), lithium hexafluorophosphate (LiPF₆),lithium borofluoride (LiBF₄), lithium hexafluoroarsenide (LiAsF₆),lithium trifluoro-metasulfonate (LiCF₃SO₃), bis-trifluoromethylsulfonylimide lithium (LiN(CF₃SO₂)₂, lithium bis(oxalato)borate (LiBOB),lithium oxalyldifluoroborate (LiBF₂C₂O₄), lithium oxalyldifluoroborate(LiBF₂C₂O₄), lithium nitrate (LiNO₃), Li-fluoroalkyl-phosphates(LiPF₃(CF₂CF₃)₃), lithium bisperfluoroethysulfonylimide (LiBETI), anionic liquid salt, sodium perchlorate (NaClO₄), potassium perchlorate(KClO₄), sodium hexafluorophosphate (NaPF₆), potassiumhexafluorophosphate (KPF₆), sodium borofluoride (NaBF₄), potassiumborofluoride (KBF₄), sodium hexafluoroarsenide, potassiumhexafluoroarsenide, sodium trifluoro-metasulfonate (NaCF₃SO₃), potassiumtrifluoro-metasulfonate (KCF₃SO₃), bis-trifluoromethyl sulfonylimidesodium (NaN(CF₃SO₂)₂), sodium trifluoromethanesulfonimide (NaTFSI),bis-trifluoromethyl sulfonylimide potassium (KN(CF₃SO₂)₂), or acombination thereof.

Ionic liquids (ILs) are a new class of purely ionic, salt-like materialsthat are liquid at unusually low temperatures. The official definitionof ILs uses the boiling point of water as a point of reference: “Ionicliquids are ionic compounds which are liquid below 100° C.”. Aparticularly useful and scientifically interesting class of ILs is theroom temperature ionic liquid (RTIL), which refers to the salts that areliquid at room temperature or below. RTILs are also referred to asorganic liquid salts or organic molten salts. An accepted definition ofan RTIL is any salt that has a melting temperature lower than ambienttemperature. Common cations of RTILs include, but not limited totetraalkylammonium, di-, tri-, or tetra-alkylimidazolium,alkylpyridinium, dialkylpyrrolidinium, dialkylpiperidinium,tetraalkylphosphonium, and trialkylsulfonium. Common anions of RTILsinclude, but not limited to, BF₄ ⁻, B(CN)₄ ⁻, CH₃BF₃ ⁻, CH₂CHBF₃ ⁻,CF₃BF₃ ⁻, C₂F₅BF₃ ⁻, n-C₃F₇BF₃ ⁻, n-C₄F₉BF₃ ⁻, PF₆ ⁻, CF₃CO₂ ⁻, CF₃SO₃⁻, N(SO₂CF₃)₂ ⁻, N(COCF₃)(SO₂CF₃)⁻, N(SO₂F)₂ ⁻, N(CN)₂ ⁻, C(CN)₃ ⁻,SCN⁻, SeCN⁻, CuCl₂ ⁻, AlCl₄ ⁻, F(HF)_(2.3) ⁻, etc.

As examples, the solvent may be selected from ethylene carbonate (EC),dimethyl carbonate (DMC), methylethyl carbonate (MEC), diethyl carbonate(DEC), ethyl propionate, methyl propionate, propylene carbonate (PC),γ-butyrolactone (γ-BL), acetonitrile (AN), ethyl acetate (EA), propylformate (PF), methyl formate (MF), toluene, xylene or methyl acetate(MA), fluoroethylene carbonate (FEC), vinylene carbonate (VC), allylethyl carbonate (AEC), 1,3-dioxolane (DOL), 1,2-dimethoxyethane (DME),tetraethylene glycol dimethylether (TEGDME), poly(ethylene glycol)dimethyl ether (PEGDME), diethylene glycol dibutyl ether (DEGDBE),2-ethoxyethyl ether (EEE), sulfone, sulfolane, room temperature ionicliquid, or a combination thereof.

In an embodiment, the rechargeable alkali metal-selenium cell mayfurther comprise a layer of protective material disposed between theanode and the porous separator, wherein the protective material is aconductor to the intended alkali metal ions (e.g. Li⁺, Na⁺, or K⁺). In apreferred embodiment, the protective material consists of a solidelectrolyte.

In an embodiment, the anode active material layer contains an anodeactive material selected from lithium metal, sodium metal, potassiummetal, a lithium metal alloy, sodium metal alloy, potassium metal alloy,a lithium intercalation compound, a sodium intercalation compound, apotassium intercalation compound, a lithiated compound, a sodiatedcompound, a potassium-doped compound, lithiated titanium dioxide,lithium titanate, lithium manganate, a lithium transition metal oxide,Li₄Ti₅O₁₂, or a combination thereof. This anode active material layercan be optionally coated on an anode current collector (such as Cufoil).

In another embodiment, the lithium-selenium battery cell is an alkalimetal ion-selenium cell (e.g. lithium ion-selenium cell, sodium-ionselenium cell, potassium ion-selenium cell) wherein the anode activematerial layer contains an anode active material selected from the groupconsisting of: (a) silicon (Si), germanium (Ge), tin (Sn), lead (Pb),antimony (Sb), bismuth (Bi), zinc (Zn), aluminum (Al), nickel (Ni),cobalt (Co), manganese (Mn), titanium (Ti), iron (Fe), and cadmium (Cd),and lithiated versions thereof; (b) alloys or intermetallic compounds ofSi, Ge, Sn, Pb, Sb, Bi, Zn, Al, or Cd with other elements, and lithiatedversions thereof, wherein said alloys or compounds are stoichiometric ornon-stoichiometric; (c) oxides, carbides, nitrides, sulfides,phosphides, selenides, and tellurides of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al,Fe, Ni, Co, Ti, Mn, or Cd, and their mixtures or composites, andlithiated versions thereof; (d) salts and hydroxides of Sn and lithiatedversions thereof; (e) carbon or graphite materials and prelithiatedversions thereof and combinations thereof.

We have discovered that the use of these types of anode active materials(instead of lithium metal foil, for instance) can eliminates thedendrite issue. The resulting battery cells are herein referred to aslithium ion selenium cells, a new breed of lithium-selenium cells.

Although in general not required, the cathode active layer of therechargeable alkali metal-selenium cell may contain a conductive fillerselected from the group consisting of electrospun nanofibers,vapor-grown carbon or graphite nanofibers, carbon or graphite whiskers,carbon nanotubes, carbon nanowires, expanded graphite flakes, metalnanowires, metal-coated nanowires or nanofibers, conductivepolymer-coated nanowires or nanofibers, and combinations thereof. In anembodiment, the conductive filler comprises a fiber selected from thegroup consisting of an electrically conductive electrospun polymerfiber, electrospun polymer nanocomposite fiber comprising a conductivefiller, nanocarbon fiber obtained from carbonization of an electrospunpolymer fiber, electrospun pitch fiber, and combinations thereof.

The mesoporous structure makes it possible to make good or fullutilization of the cathode active material (i.e. Se-containingmaterial). We have achieved a cathode active material utilizationrate >90% or even >99%. In the rechargeable alkali metal-selenium cell,the cathode contains at least 70% by weight of selenium (preferably >80%and further preferably >90%) based on the total weight of said porousgraphene structure and selenium combined

In the rechargeable alkali metal-selenium cell, the binder material (ifdesired) is selected from a resin, a conductive polymer, coal tar pitch,petroleum pitch, mesophase pitch, coke, or a derivative thereof.

In the rechargeable alkali metal-selenium cell, the cathode may furthercomprise additional selenium, selenium-containing molecule,selenium-containing compound, selenium-carbon polymer, or a combinationthereof, which is loaded before the cell is manufactured.

The presently invented cell provides a reversible specific capacity oftypically no less than 400 mAh per gram based on the total weight of theintegral cathode layer (the weights of Se, graphene material, optionalbinder, and optional conductive filler combined), not just based on theactive material weight (selenium) only. Most of the scientific papersand patent documents reported their selenium cathode specific capacitydata based on selenium weight only.

More typically and preferably, the reversible specific capacity is noless than 500 mAh per gram and often exceeds 550 or even 600 mAh pergram of entire cathode layer. The high specific capacity of thepresently invented cathode, when in combination with a lithium anode,leads to a cell specific energy of no less than 300 Wh/Kg based on thetotal cell weight including anode, cathode, electrolyte, separator, andcurrent collector weights combined. This specific energy value is notbased on the cathode active material weight or cathode layer weight only(as sometimes did in open literature or patent applications); instead,this is based on entire cell weight. In many cases, the cell specificenergy is higher than 350 Wh/Kg and, in some examples, exceeds 400Wh/kg.

These and other advantages and features of the present invention willbecome more transparent with the description of the following best modepractice and illustrative examples.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(A) SEM image of a mesoporous graphitic structure prepared byexpanding a soft carbon;

FIG. 1(B) a mesoporous graphitic structure prepared by chemicallyetching or expanding a hard carbon material;

FIG. 1(C) an expanded MCMB; and

FIG. 1(D) expanded carbon fibers.

FIG. 2 Schematic of selected procedures for producing activateddisordered carbon, oxidized or fluorinated carbon (with an expandedinter-graphene spacing), expanded carbon, and activated/expanded carbonfrom disordered carbon.

FIG. 3 Schematic of selected procedures for producing activated carbonnanotubes, oxidized or fluorinated CNTs with an expanded inter-graphenespacing, and activated/expanded CNTs from multi-walled CNTs.

FIG. 4 The charge and discharge cycling results of three Li—Se cells,one containing a presently invented cathode structure prepared by theexternal electrochemical deposition of selenium into a mesoporousstructure of chemically expanded soft carbon (C-SC), the second cellcontaining a cathode prepared by using chemical deposition of seleniumin a comparable mesoporous structure, and the third containing a cathodematerial prepared by ball-milling a mixture of Se powder and activatedcarbon powder.

FIG. 5 Ragone plots (cell power density vs. cell energy density) of twoLi metal-selenium cells: chemically etched needle coke-based cathodecontaining electrochemically deposited selenium particles (83% Se), andchemical deposition of Se into pores of the same mesoporous structure(64% Se).

FIG. 6 Ragone plots (cell power density vs. cell energy density) of 4alkali metal-selenium cells: Na—Se cell featuring a chemically expandedmesocarbon (C-MC)-based cathode containing electrochemically depositedselenium particles (70% Se), Na—Se cell featuring a C-MC-based cathodecontaining chemically deposited Se particles (70% Se), K—Se cellfeaturing a C-MC-based cathode containing electrochemically deposited Separticles (70% Se), and K—Se cell featuring a C-MC-based cathodecontaining solution-deposited Se particles (70% Se).

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

For convenience, the following discussion of preferred embodiments isprimarily based on cathodes for Li—Se cells, but the same or similarmethods are applicable to deposition of Se in the cathode for the Na—Seand K—Se cells. Examples are presented for Li—Se cells, Na—Se cells, andK—Se cells.

A. Alkali Metal-Selenium Cells (Using Lithium-Selenium Cells as anExample)

The specific capacity and specific energy of a Li—Se cell (or Na—Se, orK—Se cell) are dictated by the actual amount of selenium that can beimplemented in the cathode active layer (relative to other non-activeingredients, such as the binder resin and conductive filler) and theutilization rate of this selenium amount (i.e. the utilizationefficiency of the cathode active material or the actual proportion of Sthat actively participates in storing and releasing lithium ions). Ahigh-capacity and high-energy Li—Se requires a high amount of Se in thecathode active layer (i.e. relative to the amounts of non-activematerials, such as the binder resin, conductive additive, and othermodifying or supporting materials) and a high Se utilizationefficiency). The present invention provides such a cathode active layerand a method of producing such a cathode active layer, which is apre-selenized active cathode layer. This method comprises the followingfour steps, (a)-(d):

-   -   a) Preparing an integral layer of mesoporous structure (a porous        sheet, paper, web, film, fabric, non-woven, mat, aggregate, or        foam) having mesopores, 0.5-50 nm in size) of a carbon,        graphite, metal, or polymer material having massive surfaces to        accommodate Se thereon with a specific surface area from 100 to        3,200 m²/g (these surfaces must be accessible to electrolyte).        The mesoporous structure has a specific surface area        preferably >500 m²/g and more preferably >750 m²/g, and most        preferably >1,000 m²/g.    -   b) Preparing an electrolyte comprising a solvent (preferably        non-aqueous solvent, such as organic solvent and or ionic        liquid) and at least a selenium source dissolved or dispersed in        the solvent;    -   c) Preparing an anode; and    -   d) Bringing the integral layer of mesoporous structure and the        anode in ionic contact with the electrolyte (e.g. by immersing        all these components in a chamber that is external to the        intended Li—Se cell, or encasing these three components inside        the Li—Se cell) and imposing an electric current between the        anode and the integral layer of mesoporous structure (serving as        a cathode) with a sufficient current density for a sufficient        period of time to electrochemically deposit nanoscaled selenium        or metal selenide particles or coating in the mesopores to form        the pre-selenized active cathode layer.

The integral layer of a mesoporous structure comprises primarily acarbon, graphite, metal, or conductive polymer selected from chemicallyetched or expanded soft carbon, chemically etched or expanded hardcarbon, exfoliated activated carbon, chemically etched or expandedcarbon black, chemically etched multi-walled carbon nanotube,nitrogen-doped carbon nanotube, boron-doped carbon nanotube, chemicallydoped carbon nanotube, ion-implanted carbon nanotube, chemically treatedmulti-walled carbon nanotube with an inter-planar separation no lessthan 0.5 nm, chemically expanded carbon nanofiber, chemically activatedcarbon nanotube, chemically treated carbon fiber, chemically activatedgraphite fiber, chemically activated carbonized polymer fiber,chemically treated coke, activated mesophase carbon, mesoporous carbon,electrospun conductive nanofiber, vapor-grown carbon or graphitenanofiber, carbon or graphite whisker, carbon nanotube, carbon nanowire,metal nanowire, metal-coated nanowire or nanofiber, conductivepolymer-coated nanowire or nanofiber, or a combination thereof.Particles and/or fibrils of this material, when packed into an integralelectrode layer of mesoporous structure must still exhibit a specificsurface area >100 m²/g that this in direct contact with the electrolyte.The mesopores must be accessible to the electrolyte.

The layer of mesoporous structure may optionally contain 0-49%(preferably 0-30%, more preferably 0-30%, and further preferably 0-10%)by weight of selenium or selenium-containing compound pre-loadedtherein, based on the weights of all ingredients in the layer prior tothe step (d) of depositing selenium coating or particles in themesopores. Preferably, zero (0%) selenium or sulfur-containing compoundis pre-loaded into the porous graphene structure since this pre-loadedmaterial, if not done properly, can negatively impact the subsequentpre-selenization step.

The Se particles or coating have a thickness or diameter smaller than 20nm (preferably <10 nm, more preferably <5 nm, and further preferably <3nm) and wherein the nanoscaled selenium particles or coating occupy aweight fraction of at least 70% (preferably >80%, more preferably >90%,and most preferably >95%) based on the total weights of the seleniumparticles or coating and the graphene material combined. It isadvantageous to deposit as much Se as possible yet still maintainultra-thin thickness or diameter of the Se coating or particles(e.g. >80% and <3 nm; >90% and <5 nm; and >95% and <10 nm, etc.).

B. Production of Various Mesoporous Structures

The following types of porous structures are found to be particularlysuitable for use to support and protect the sulfur coating or particles:a porous sheet, paper, web, film, fabric, non-woven, mat, aggregate, orfoam of a carbon or graphite material that has been expanded, activated,chemically treated, and/or expanded. This porous structure can containchemically etched or expanded soft carbon, chemically etched or expandedhard carbon, exfoliated activated carbon, chemically etched or expandedcarbon black, chemically etched multi-walled carbon nanotube,nitrogen-doped carbon nanotube, boron-doped carbon nanotube, chemicallydoped carbon nanotube, ion-implanted carbon nanotube, chemically treatedmulti-walled carbon nanotube with an inter-graphene planar separation noless than 0.4 nm, chemically expanded carbon nanofiber, chemicallyactivated or expanded carbon nanotube, carbon fiber, graphite fiber,carbonized polymer fiber, coke, mesophase carbon, or a combinationthereof. The expanded spacing is preferably >0.6 nm, morepreferably >1.0 nm, and most preferably from 1.0 nm to 3.0 nm.

Alternatively, the mesoporous structure may contain a porous,electrically conductive material selected from metal foam, carbon-coatedmetal foam, graphene-coated metal foam, metal web or screen,carbon-coated metal web or screen, graphene-coated metal web or screen,perforated metal sheet, carbon-coated porous metal sheet,graphene-coated porous metal sheet, metal fiber mat, carbon-coatedmetal-fiber mat, graphene-coated metal-fiber mat, metal nanowire mat,carbon-coated metal nanowire mat, graphene-coated metal nanowire mat,surface-passivated porous metal, porous conductive polymer film,conductive polymer nanofiber mat or paper, conductive polymer foam,carbon foam, carbon aerogel, carbon xerogel, or a combination thereof.These porous and electrically conductive materials are capable ofaccommodating sulfur in their pores and, in many cases, capable ofprotecting the sulfur coating or particles from getting dissolved in aliquid electrolyte, in addition to providing a 3-D network ofelectron-conducting paths. For the purpose of defining the claims, theinstant cathode does not contain those isolated graphene sheets orplatelets not supported on metal, carbon, ceramic, or polymer fibers orfoams.

Conductive polymer nanofiber mats can be readily produced byelectrospinning of a conductive polymer, which can be an intrinsicallyconductive (conjugate-chain) polymer or a conductive filler-filledpolymer. Electrospinning is well-known in the art. The production ofcarbon foam, carbon aerogel, or carbon xerogel is also well-known in theart.

Particularly useful metal foams include copper foam, stainless steelfoam, nickel foam, titanium foam, and aluminum foam. The fabrication ofmetal foams is well known in the art and a wide variety of metal foamsare commercially available. Preferably, the surfaces of metallic foamsare coated with a thin layer of carbon or graphene because carbon andgraphene are more electrochemically inert and will not get dissolvedduring the charge/discharge cycles of the cell. Hence, carbon-coatedmetal foam, graphene-coated metal foam, carbon-coated metal web orscreen, graphene-coated metal web or screen, carbon-coated porous metalsheet, graphene-coated porous metal sheet, carbon-coated metal-fibermat, graphene-coated metal-fiber mat, carbon-coated metal nanowire mat,and graphene-coated metal nanowire mat are preferred current collectormaterials for use in the rechargeable lithium cell. Also particularlyuseful are carbon foam, carbon aerogel, and carbon xerogel. These foamsmay be reinforced with a binder resin, conductive polymer, or CNTs tomake a porous structure of good structural integrity.

In one preferred embodiment, highly porous graphitic or carbonaceousmaterials may be used to make a conductive and protective backboneporous structure prior to impregnating the resulting porous structurewith sulfur. In this approach, particles of these materials may bebonded by a binder to form a porous structure of good structuralintegrity.

In another possible route, porous graphitic or carbonaceous materialparticles, along with a resin binder, may be coated onto surfaces of ahighly porous metal framework with large pores, such as a metal foam,web, or screen, which serves as a backbone for a mesoporous structure.The combined hybrid structure is preferably very porous with a specificsurface area significantly greater than 100 m²/g.

The carbonaceous or graphitic material may be selected from chemicallytreated graphite with an inter-graphene planar separation no less than0.5 nm (preferably greater than 0.6 nm, more preferably greater than 1.0nm) which is not exfoliated, soft carbon (preferably, chemically etchedor expanded soft carbon), hard carbon (preferably, chemically etched orexpanded hard carbon), activated carbon (preferably, exfoliatedactivated carbon), carbon black (preferably, chemically etched orexpanded carbon black), chemically expanded multi-walled carbonnanotube, chemically expanded carbon fiber or nanofiber, or acombination thereof. These carbonaceous or graphitic materials have onething in common; they all have mesoscaled pores, enabling entry ofelectrolyte to access their interior planes of hexagonal carbon atoms.

In one preferred embodiment, the mesoporous carbonaceous material may beproduced by using the following recommended procedures:

-   -   (A) dispersing or immersing a graphitic or carbonaceous material        (e.g., powder of mesophase carbon, mesocarbon micro bead (MCMB),        soft carbon, hard carbon, coke, polymeric carbon (carbonized        resin), activated carbon (AC), carbon black (CB), multi-walled        carbon nanotube (MWCNT), carbon nanofiber (CNF), carbon or        graphite fiber, mesophase pitch fiber, and the like) in a        mixture of an intercalant and/or an oxidant (e.g., concentrated        sulfuric acid and nitric acid) and/or a fluorinating agent to        obtain a carbon intercalation compound (CIC), graphite fluoride        (GF), or chemically etched/treated carbon material; and        optionally    -   (B) exposing the resulting CIC, GF, or chemically etched/treated        carbon material to a thermal treatment, preferably in a        temperature range of 150-600° C. for a short period of time        (typically 15 to 60 seconds) to obtain expanded carbon.        Alternatively, after step (A) above, the resulting CIC, GF, or        chemically etched/treated carbon material is subjected to        repeated rinsing/washing to remove excess chemical. The rinsed        products are then subjected to a drying procedure to remove        water. The dried CIC, GF, chemically treated CB, chemically        treated AC, chemically treated MWCNT, chemically treated CNF,        chemically treated carbon/graphite/pitch fiber can be used as a        cathode active material of the presently invented high-capacity        cell. These chemically treated carbonaceous or graphitic        materials can be further subjected to a heat treatment at a        temperature preferably in the range of 150-600° C. for the        purposes of creating mesoscaled pores (0.5-50 nm) to enable the        interior structure being accessed by electrolyte. It may be        noted that these interior graphene planes remain stacked and        interconnected with one another, but the above-described        chemical/thermal treatments facilitate direct access of these        interior graphene planes by the electrolyte.

The broad array of carbonaceous materials, such as a soft carbon, hardcarbon, polymeric carbon (or carbonized resin), mesophase carbon, coke,carbonized pitch, carbon black, activated carbon, or partiallygraphitized carbon, are commonly referred to as the disordered carbonmaterial. A disordered carbon material is typically formed of two phaseswherein a first phase is small graphite crystal(s) or small stack(s) ofgraphite planes (with typically up to 10 graphite planes or aromaticring structures overlapped together to form a small ordered domain) anda second phase is non-crystalline carbon, and wherein the first phase isdispersed in the second phase or bonded by the second phase. The secondphase is made up of mostly smaller molecules, smaller aromatic rings,defects, and amorphous carbon. Typically, the disordered carbon ishighly porous (e.g., exfoliated activated carbon), or present in anultra-fine powder form (e.g. chemically etched carbon black) havingnanoscaled features (e.g. having mesoscaled pores and, hence, a highspecific surface area).

Soft carbon refers to a carbonaceous material composed of small graphitecrystals wherein the orientations of these graphite crystals or stacksof graphene planes inside the material are conducive to further mergingof neighboring graphene sheets or further growth of these graphitecrystals or graphene stacks using a high-temperature heat treatment.This high temperature treatment is commonly referred to asgraphitization and, hence, soft carbon is said to be graphitizable.

Hard carbon refers to a carbonaceous material composed of small graphitecrystals wherein these graphite crystals or stacks of graphene planesinside the material are not oriented in a favorable direction (e.g.nearly perpendicular to each other) and, hence, are not conducive tofurther merging of neighboring graphene planes or further growth ofthese graphite crystals or graphene stacks (i.e., not graphitizable).

Carbon black (CB) (including acetylene black, AB) and activated carbon(AC) are typically composed of domains of aromatic rings or smallgraphene sheets, wherein aromatic rings or graphene sheets in adjoiningdomains are somehow connected through some chemical bonds in thedisordered phase (matrix). These carbon materials are commonly obtainedfrom thermal decomposition (heat treatment, pyrolyzation, or burning) ofhydrocarbon gases or liquids, or natural products (wood, coconut shells,etc). These materials per se (without chemical/thermal treatments asdescribed above) are not good candidate cathode materials for thepresently invented high-capacity Li-ion cells. Hence, preferably, theyare subjected to further chemical etching or chemical/thermalexfoliation to form a mesoporous structure having a pore size in therange of 0.5-50 nm (preferably 2-10 nm). These mesoscaled pores enablethe liquid electrolyte to enter the pores and access the graphene planesinside individual particles of these carbonaceous materials.

The preparation of polymeric carbons by simple pyrolysis of polymers orpetroleum/coal tar pitch materials has been known for approximatelythree decades. When polymers such as polyacrylonitrile (PAN), rayon,cellulose and phenol formaldehyde were heated above 300° C. in an inertatmosphere they gradually lost most of their non-carbon contents. Theresulting structure is generally referred to as a polymeric carbon.Depending upon the heat treatment temperature (HTT) and time, polymericcarbons can be made to be insulating, semi-conducting, or conductingwith the electric conductivity range covering approximately 12 orders ofmagnitude. This wide scope of conductivity values can be furtherextended by doping the polymeric carbon with electron donors oracceptors. These characteristics uniquely qualify polymeric carbons as anovel, easy-to-process class of electroactive materials whose structuresand physical properties can be readily tailor-made.

Polymeric carbons can assume an essentially amorphous structure or havemultiple graphite crystals or stacks of graphene planes dispersed in anamorphous carbon matrix. Depending upon the HTT used, variousproportions and sizes of graphite crystals and defects are dispersed inan amorphous matrix. Various amounts of two-dimensional condensedaromatic rings or hexagons (precursors to graphene planes) can be foundinside the microstructure of a heat treated polymer such as a PAN fiber.An appreciable amount of small-sized graphene sheets are believed toexist in PAN-based polymeric carbons treated at 300-1,000° C. Thesespecies condense into wider aromatic ring structures (larger-sizedgraphene sheets) and thicker plates (more graphene sheets stackedtogether) with a higher HTT or longer heat treatment time (e.g., >1,500°C.). These graphene platelets or stacks of graphene sheets (basalplanes) are dispersed in a non-crystalline carbon matrix. Such atwo-phase structure is a characteristic of some disordered carbonmaterial.

There are several classes of precursor materials to the disorderedcarbon materials of the instant patent application. For instance, thefirst class includes semi-crystalline PAN in a fiber form. As comparedto phenolic resin, the pyrolized PAN fiber has a higher tendency todevelop small crystallites that are dispersed in a disordered matrix.The second class, represented by phenol formaldehyde, is a moreisotropic, essentially amorphous and highly cross-linked polymer. Thethird class includes petroleum and coal tar pitch materials in bulk orfiber forms. The precursor material composition, heat treatmenttemperature (HTT), and heat treatment time (Htt) are three parametersthat govern the length, width, thickness (number of graphene planes in agraphite crystal), and chemical composition of the resulting disorderedcarbon materials.

In the present investigation, PAN fibers were subjected to oxidation at200-350° C. while under a tension, and then partial or completecarbonization at 350-1,500° C. to obtain polymeric carbons with variousnanocrystalline graphite structures (graphite crystallites). Selectedsamples of these polymeric carbons were further heat-treated at atemperature in the range of 1,500-2,000° C. to partially graphitize thematerials, but still retaining a desired amount of amorphous carbon (noless than 10%). Phenol formaldehyde resin and petroleum and coal tarpitch materials were subjected to similar heat treatments in atemperature range of 500 to 1,500° C. The disordered carbon materialsobtained from PAN fibers or phenolic resins are preferably subjected toa chemical etching/expanding treatment using a process commonly used toproduce activated carbon (e.g., treated in a KOH melt at 900° C. for 1-5hours). This chemical treatment is intended for making the disorderedcarbon mesoporous, enabling electrolyte to reach the edges or surfacesof the constituent aromatic rings after a battery cell is made. Such anarrangement enables the lithium ions in the liquid electrolyte toreadily attach onto exposed graphene planes or edges without having toundergo significant solid-state diffusion.

Certain grades of petroleum pitch or coal tar pitch may be heat-treated(typically at 250-500° C.) to obtain a liquid crystal-type, opticallyanisotropic structure commonly referred to as mesophase. This mesophasematerial can be extracted out of the liquid component of the mixture toproduce isolated mesophase particles or spheres, which can be furthercarbonized and graphitized.

In general, the cathode active material (including the porous backbonestructure and S lodged in the pores) as a whole also preferably form amesoporous structure with a desired amount of mesoscaled pores (0.5-50nm, preferably 2-10 nm) to allow the entry of electrolyte. This isadvantageous because these pores enable a great amount of surface areasto be in physical contact with electrolyte and capable of capturing Seprecipitated from the electrolyte during the subsequent electrochemicaldeposition and capturing/releasing lithium (sodium or potassium) from/tothe electrolyte during subsequent battery charges/discharges. Thesesurface areas of the cathode active material as a whole are typicallyand preferably >100 m²/g, more preferably >500 m²/g, further morepreferably >1,000 m²/g, and most preferably >1,500 m²/g.

C. Deposition of Selenium in Mesopores

Once a layer of mesoporous structure (e.g. a porous sheet, paper, web,film, fabric, non-woven, mat, aggregate, or foam having mesopores,0.5-50 nm in size) is prepared, this layer can be immersed in anelectrolyte (preferably liquid electrolyte), which comprises a solventand a selenium source dissolved or dispersed in the solvent. This layerbasically serves as a cathode in an external electrochemical depositionchamber or a cathode in an intended alkali metal-sulfur cell (encasedinside the packaging or casing of a battery).

Subsequently, an anode layer is also immersed in the chamber, or encasedinside a battery cell. Any conductive material can be used as an anodematerial, but preferably this layer contains some lithium (or sodium orpotassium). In such an arrangement, the integral layer of mesoporousstructure and the anode are in ionic contact with the electrolyte. Anelectric current is then supplied between the anode and the integrallayer of mesoporous structure (serving as a cathode) with a sufficientcurrent density for a sufficient period of time to electrochemicallydeposit nanoscaled selenium particles or coating in the mesopores toform the pre-selenized active cathode layer. The required currentdensity depends upon the desired speed of deposition and uniformity ofthe deposited material.

This current density can be readily adjusted to deposit Se particles orcoating that have a thickness or diameter smaller than 20 nm (preferably<10 nm, more preferably <5 nm, and further preferably <3 nm). Theresulting nanoscaled selenium particles or coating typically andpreferably occupy a weight fraction of at least 70% (preferably >80%,more preferably >90%, and most preferably >95%) based on the totalweights of the selenium particles or coating and the mesoporousstructure combined.

In one preferred embodiment, the selenium source is selected fromM_(x)Se_(y), wherein x is an integer from 1 to 3 and y is an integerfrom 1 to 10, and M is a metal element selected from an alkali metal, analkaline metal selected from Mg or Ca, a transition metal, a metal fromgroups 13 to 17 of the periodic table, or a combination thereof. In adesired embodiment, the metal element M is selected from Li, Na, K, Mg,Zn, Cu, Ti, Ni, Co, Fe, or Al. In a particularly desired embodiment,M_(x)Se_(y) is selected from Li₂Se₆, Li₂Se₇, Li₂Se₈, Li₂Se₉, Li₂Se₁₀,Na₂Se₆, Na₂Se₇, Na₂Se₈, Na₂Se₉, Na₂Se₁₀, K₂Se₆, K₂Se₇, K₂Se₈, K₂Se₉, orK₂Se₁₀.

In one embodiment, the anode comprises an anode active material selectedfrom an alkali metal, an alkaline metal, a transition metal, a metalfrom groups 13 to 17 of the periodic table, or a combination thereof.This anode can be the same anode intended for inclusion in a Li—Se cell.

The solvent may be selected from 1,3-dioxolane (DOL),1,2-dimethoxyethane (DME), tetraethylene glycol dimethylether (TEGDME),poly(ethylene glycol) dimethyl ether (PEGDME), diethylene glycol dibutylether (DEGDBE), 2-ethoxyethyl ether (EEE), sulfone, sulfolane, ethylenecarbonate (EC), dimethyl carbonate (DMC), methylethyl carbonate (MEC),diethyl carbonate (DEC), ethyl propionate, methyl propionate, propylenecarbonate (PC), γ-butyrolactone (γ-BL), acetonitrile (AN), ethyl acetate(EA), propyl formate (PF), methyl formate (MF), toluene, xylene, methylacetate (MA), fluoroethylene carbonate (FEC), vinylene carbonate (VC),allyl ethyl carbonate (AEC), a hydrofluoroether, a room temperatureionic liquid solvent, or a combination thereof.

For the purpose of internal electrochemical deposition of Se in themesopores of a cathode layer in a cell, the electrolyte may furthercomprise a metal salt selected from lithium perchlorate (LiClO₄),lithium hexafluorophosphate (LiPF₆), lithium borofluoride (LiBF₄),lithium hexafluoroarsenide (LiAsF₆), lithium trifluoro-metasulfonate(LiCF₃SO₃), bis-trifluoromethyl sulfonylimide lithium (LiN(CF₃SO₂)₂),lithium bis(oxalato)borate (LiBOB), lithium oxalyldifluoroborate(LiBF₂C₂O₄), lithium oxalyldifluoroborate (LiBF₂C₂O₄), lithium nitrate(LiNO₃), Li-fluoroalkyl-phosphate (LiPF₃(CF₂CF₃)₃), lithiumbisperfluoro-ethysulfonylimide (LiBETI), lithiumbis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide,lithium trifluoromethanesulfonimide (LiTFSI), an ionic liquid-basedlithium salt, sodium perchlorate (NaClO₄), potassium perchlorate(KClO₄), sodium hexafluorophosphate (NaPF₆), potassiumhexafluorophosphate (KPF₆), sodium borofluoride (NaBF₄), potassiumborofluoride (KBF₄), sodium hexafluoroarsenide, potassiumhexafluoroarsenide, sodium trifluoro-metasulfonate (NaCF₃SO₃), potassiumtrifluoro-metasulfonate (KCF₃SO₃), bis-trifluoromethyl sulfonylimidesodium (NaN(CF₃SO₂)₂), sodium trifluoromethanesulfonimide (NaTFSI),bis-trifluoromethyl sulfonylimide potassium (KN(CF₃SO₂)₂), or acombination thereof.

In one preferred embodiment, as previously stated above, theelectrochemical deposition is conducted before the cathode active layeris incorporated into a lithium-selenium (Li—Se) battery cell. In otherwords, the anode, the electrolyte, and the integral layer of mesoporousstructure (serving as a cathode layer) are positioned in an externalcontainer outside of a lithium-selenium cell. The needed apparatus issimilar to an electroplating system. The step of electrochemicallydepositing nanoscaled selenium particles or coating in the mesopores isconducted outside the lithium-selenium cell and prior to the batterycell fabrication. After this selenium deposition is completed, thepre-selenized integral layer of mesoporous structure is thenincorporated into the lithium-selenium cell.

In another embodiment, the anode, the electrolyte, and the integrallayer of mesoporous structure are disposed inside a lithium-seleniumcell. In other words, the battery cell itself is an electrochemicaldeposition system for pre-selenization of the cathode and the step ofelectrochemically depositing nanoscaled selenium particles or coating inthe mesopores is conducted after the lithium-selenium cell isfabricated. This electrochemical deposition procedure is conductedduring the first charge cycle of the Li—Se cell.

After an extensive and in-depth research effort, we have come to realizethat such a pre-selenization surprisingly solves several most criticalissues associated with current Li—Se cells. For instance, this methodenables the selenium to be deposited in a thin coating or ultra-fineparticle form, thus, providing ultra-short lithium ion diffusion pathsand, hence, ultra-fast reaction times for fast battery charges anddischarges. This is achieved while maintaining a relatively highproportion of selenium (the active material responsible for storinglithium) and, thus, high specific lithium storage capacity of theresulting cathode active layer in terms of high specific capacity(mAh/g, based on the total weight of the cathode layer, including themasses of the active material, Se, supporting graphene sheets, binderresin, and conductive filler).

It is of significance to note that one might be able to use a prior artprocedure to deposit small Se particles, but not a high Se proportion,or to achieve a high proportion but only in large particles or thickfilm form. But, the prior art procedures have not been able to achieveboth small Se particles and high Se proportion at the same time. This iswhy it is such an unexpected and highly advantageous thing to obtain ahigh selenium loading and yet, concurrently, maintaining anultra-thin/small thickness/diameter of selenium. This has not beenpossible with any prior art selenium loading techniques. For instance,we have been able to deposit nanoscaled selenium particles or coatingthat occupy a >90% weight fraction of the cathode layer and yetmaintaining a coating thickness or particle diameter <3 nm. This isquite a feat in the art of lithium-selenium batteries. As anotherexample, we have achieved a >95% Se loading at an average Se coatingthickness of 4.5-7.1 nm.

Electrochemists or materials scientists in the art of Li—Se batterieswould expect that a greater amount of conductive supporting materials(hence, a smaller amount of Se) in the cathode active layer would berequired in order to achieve a better utilization of Se, particularlyunder high charge/discharge rate conditions. Contrary to theseexpectations, we have observed that the key to achieving a high Seutilization efficiency is minimizing the Se coating thickness or Separticle size and is independent of the amount of Se loaded into thecathode provided the Se coating or particle thickness/diameter is smallenough (e.g. <10 nm, or even better if <5 nm). The problem here is thatit has not been possible in prior art to maintain a thin Se coating orsmall particle size if Se is higher than 50% by weight. Here we havefurther surprisingly observed that the key to enabling a high specificcapacity at the cathode under high rate conditions is to maintain a highSe loading and still keep the Se coating or particle size as small aspossible, and this is accomplished by using the presently inventedpre-selenization (selenium pre-loading) method.

The electrons coming from or going out through the external load orcircuit must go through the conductive additives (in a conventionalselenium cathode) or a conductive framework (e.g. the mesoporousstructure as herein disclosed) to reach the cathode active material.Since the cathode active material (e.g. selenium or lithiumpolyselenide) is a poor electronic conductor, the active materialparticle or coating must be as thin as possible to reduce the requiredelectron travel distance.

Furthermore, the cathode in a conventional Li—Se cell typically has lessthan 70% by weight of selenium in a composite cathode composed ofselenium and the conductive additive/support. Even when the seleniumcontent in the prior art composite cathode reaches or exceeds 70% byweight, the specific capacity of the composite cathode is typicallysignificantly lower than what is expected based on theoreticalpredictions. For instance, the theoretical specific capacity of seleniumis 675 mAh/g. A composite cathode composed of 70% selenium (Se) and 30%carbon black (CB), without any binder, should be capable of storing upto 675×70%=472.5 mAh/g. Unfortunately, the observed specific capacity istypically less than 75% or 354 mAh/g (often less than 50% or 237 mAh/gin this example) of what could be achieved. In other words, the activematerial utilization rate is typically less than 75% (or even <50%).This has been a major issue in the art of Li—Se cells and there has beenno solution to this problem. Most surprisingly, the implementation ofmassive surfaces associated with a mesoporous structure as a conductivesupporting material for selenium or lithium polyselenide has made itpossible to achieve an active material utilization rate oftypically >>80%, more often greater than 90%, and, in many cases, closeto 95%-99%.

Still another unexpected result of the instant method is the observationthat thinner Se coating leads to more stable charge/discharge cyclingwith significantly reduced shuttling effect that has been along-standing impediment to full commercialization of Li—Se batteries.We overcome this problem yet, at the same time, achieving a highspecific capacity. In just about all the prior art Li—Se cells, a higherSe loading leads to a faster capacity decay.

The shuttling effect is related to the tendency for selenium or lithiumpolyselenide that forms at the cathode to get dissolved in the solventand for the dissolved species to migrate from the cathode to the anode,where they irreversibly react with lithium to form species that preventselenide from returning back to the cathode during the subsequentdischarge operation of the Li—Se cell (the detrimental shuttlingeffect). It seems that the mesopores in the presently inventedmesoporous structure have been able to prevent or reduce such adissolution and migration issue.

The electrolytic salts to be incorporated into a non-aqueous electrolytemay be selected from a lithium salt such as lithium perchlorate(LiClO₄), lithium hexafluorophosphate (LiPF₆), lithium borofluoride(LiBF₄), lithium hexafluoroarsenide (LiAsF₆), lithiumtrifluoro-metasulfonate (LiCF₃SO₃), bis-trifluoromethyl sulfonylimidelithium (LiN(CF₃SO₂)₂), lithium bis(oxalato)borate (LiBOB), lithiumoxalyldifluoroborate (LiBF₂C₂O₄), lithium oxalyldifluoroborate(LiBF₂C₂O₄), lithium nitrate (LiNO₃), lithium-fluoroalkyl-phosphates(LiPF3(CF₂CF₃)₃), lithium bisperfluoroethysulfonylimide (LiBETI), anionic liquid salt. Among them, LiPF₆, LiBF₄ and LiN(CF₃SO₂)₂ arepreferred. The content of aforementioned electrolytic salts in thenon-aqueous solvent is preferably 0.5 to 3.0 M (mol/L) at the cathodeside and 3.0 to >10 M at the anode side.

The ionic liquid is composed of ions only. Ionic liquids are low meltingtemperature salts that are in a molten or liquid state when above adesired temperature. For instance, a salt is considered as an ionicliquid if its melting point is below 100° C. If the melting temperatureis equal to or lower than room temperature (25° C.), the salt isreferred to as a room temperature ionic liquid (RTIL). The IL salts arecharacterized by weak interactions, due to the combination of a largecation and a charge-delocalized anion. This results in a low tendency tocrystallize due to flexibility (anion) and asymmetry (cation).

A typical and well-known ionic liquid is formed by the combination of a1-ethyl-3-methylimidazolium (EMI) cation and anN,N-bis(trifluoromethane)sulfonamide (TFSI) anion. This combinationgives a fluid with an ionic conductivity comparable to many organicelectrolyte solutions and a low decomposition propensity and low vaporpressure up to ˜300-400° C. This implies a generally low volatility andnon-flammability and, hence, a much safer electrolyte for batteries.

Ionic liquids are basically composed of organic ions that come in anessentially unlimited number of structural variations owing to thepreparation ease of a large variety of their components. Thus, variouskinds of salts can be used to design the ionic liquid that has thedesired properties for a given application. These include, among others,imidazolium, pyrrolidinium and quaternary ammonium salts as cations andbis(trifluoromethanesulfonyl) imide, bis(fluorosulfonyl)imide, andhexafluorophosphate as anions. Based on their compositions, ionicliquids come in different classes that basically include aprotic, proticand zwitterionic types, each one suitable for a specific application.

Common cations of room temperature ionic liquids (RTILs) include, butnot limited to, tetraalkylammonium, di-, tri-, andtetra-alkylimidazolium, alkylpyridinium, dialkyl-pyrrolidinium,dialkylpiperidinium, tetraalkylphosphonium, and trialkylsulfonium.Common anions of RTILs include, but not limited to, BF₄ ⁻, B(CN)₄ ⁻,CH₃BF₃ ⁻, CH2CHBF₃ ⁻, CF₃BF₃ ⁻, C₂F₅BF₃ ⁻, n-C₃F₇BF₃ ⁻, n-C₄F₉BF₃ ⁻, PF₆⁻, CF₃CO₂ ⁻, CF₃SO₃ ⁻, N(SO₂CF₃)₂ ⁻, N(COCF₃)(SO₂CF₃)⁻, N(SO₂F)₂ ⁻,N(CN)₂ ⁻, C(CN)₃ ⁻, SCN⁻, SeCN⁻, CuCl₂ ⁻, AlCl₄ ⁻, F(HF)_(2.3) ⁻, etc.Relatively speaking, the combination of imidazolium- or sulfonium-basedcations and complex halide anions such as AlCl₄ ⁻, BF₄ ⁻, CF₃CO₂ ⁻,CF₃SO₃ ⁻, NTf₂ ⁻, N(SO₂F)₂ ⁻, or F(HF)_(2.3) ⁻ results in RTILs withgood working conductivities.

RTILs can possess archetypical properties such as high intrinsic ionicconductivity, high thermal stability, low volatility, low (practicallyzero) vapor pressure, non-flammability, the ability to remain liquid ata wide range of temperatures above and below room temperature, highpolarity, high viscosity, and wide electrochemical windows. Theseproperties, except for the high viscosity, are desirable attributes whenit comes to using an RTIL as an electrolyte ingredient (a salt and/or asolvent) in a Li—Se cell.

In one embodiment, the cathode layer may be pre-loaded with up to 30%(preferably <15% and more preferably <10%) of an active material(selenium or lithium poly selenide) prior to the cathode layerfabrication. In yet another embodiment, the cathode layer can contain aconductive filler, such as carbon black (CB), acetylene black (AB),graphite particles, activated carbon, mesoporous carbon, mesocarbonmicro bead (MCMB), carbon nanotube (CNT), carbon nanofiber (CNF), carbonfiber, or a combination thereof.

The anode active material may contain, as an example, lithium metal foilor a high-capacity Si, Sn, or SnO₂ capable of storing a great amount oflithium. The cathode active material may contain pure selenium (if theanode active material contains lithium), lithium polyselenide, or anyselenium-containing compound, molecule, or polymer. If the cathodeactive material includes lithium-containing species (e.g. lithiumpolyselenide) when the cell is made, the anode active material can beany material capable of storing a large amount of lithium (e.g. Si, Ge,Sn, SiO, SnO₂, etc.).

At the anode side, when lithium metal is used as the sole anode activematerial in a Li—Se cell, there is concern about the formation oflithium dendrites, which could lead to internal shorting and thermalrunaway. Herein, we have used two approaches, separately or incombination, to address this dendrite formation issue: one involving theuse of a high-concentration electrolyte at the anode side and the otherthe use of a nanostructure composed of conductive nanofilaments. For thelatter, multiple conductive nanofilaments are processed to form anintegrated aggregate structure, preferably in the form of a closelypacked web, mat, or paper, characterized in that these filaments areintersected, overlapped, or somehow bonded (e.g., using a bindermaterial) to one another to form a network of electron-conductingpathways. The integrated structure has substantially interconnectedpores to accommodate electrolyte. The nanofilament may be selected from,as examples, a carbon nanofiber (CNF), graphite nanofiber (GNF), carbonnanotube (CNT), metal nanowire (MNW), conductive nanofibers obtained byelectrospinning, conductive electrospun composite nanofibers, nanoscaledgraphene platelet (NGP), or a combination thereof. The nanofilaments maybe bonded by a binder material selected from a polymer, coal tar pitch,petroleum pitch, mesophase pitch, coke, or a derivative thereof.

Nanofibers may be selected from the group consisting of an electricallyconductive electrospun polymer fiber, electrospun polymer nanocompositefiber comprising a conductive filler, nanocarbon fiber obtained fromcarbonization of an electrospun polymer fiber, electrospun pitch fiber,and combinations thereof. For instance, a nanostructured electrode canbe obtained by electrospinning of polyacrylonitrile (PAN) into polymernanofibers, followed by carbonization of PAN. It may be noted that someof the pores in the structure, as carbonized, are greater than 100 nmand some smaller than 100 nm.

The presently invented cathode active layer may be incorporated in oneof at least four broad classes of rechargeable lithium metal cells (or,similarly, for sodium metal or potassium metal cells):

-   -   (A) Lithium metal-selenium with a conventional anode        configuration: The cell contains an optional cathode current        collector, a presently invented cathode layer, a        separator/electrolyte, and an anode current collector. Potential        dendrite formation may be overcome by using the        high-concentration electrolyte at the anode.    -   (B) Lithium metal-selenium cell with a nanostructured anode        configuration: The cell contains an optional cathode current        collector, a cathode herein invented, a separator/electrolyte,        an optional anode current collector, and a nanostructure to        accommodate lithium metal that is deposited back to the anode        during a charge or re-charge operation. This nanostructure (web,        mat, or paper) of nanofilaments provide a uniform electric field        enabling uniform Li metal deposition, reducing the propensity to        form dendrites. This configuration can provide a dendrite-free        cell for a long and safe cycling behavior.    -   (C) Lithium ion-selenium cell with a conventional anode: For        instance, the cell contains an anode composed of anode active        graphite particles bonded by a binder, such as polyvinylidene        fluoride (PVDF) or styrene-butadiene rubber (SBR). The cell also        contains a cathode current collector, a cathode of the instant        invention, a separator/electrolyte, and an anode current        collector; and    -   (D) Lithium ion-selenium cell with a nanostructured anode: For        instance, the cell contains a web of nanofibers coated with Si        coating or bonded with Si nanoparticles. The cell also contains        an optional cathode current collector, a cathode herein        invented, a separator/electrolyte, and an anode current        collector. This configuration provides an ultra-high capacity,        high energy density, and a safe and long cycle life.

In the lithium-ion selenium cell (e.g. as described in (C) and (D)above), the anode active material can be selected from a wide range ofhigh-capacity materials, including (a) silicon (Si), germanium (Ge), tin(Sn), lead (Pb), antimony (Sb), bismuth (Bi), zinc (Zn), aluminum (Al),nickel (Ni), cobalt (Co), manganese (Mn), titanium (Ti), iron (Fe), andcadmium (Cd), and lithiated versions thereof; (b) alloys orintermetallic compounds of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, or Cd withother elements, and lithiated versions thereof, wherein said alloys orcompounds are stoichiometric or non-stoichiometric; (c) oxides,carbides, nitrides, sulfides, phosphides, selenides, and tellurides ofSi, Ge, Sn, Pb, Sb, Bi, Zn, Al, Fe, Ni, Co, Ti, Mn, or Cd, and theirmixtures or composites, and lithiated versions thereof; (d) salts andhydroxides of Sn and lithiated versions thereof; (e) carbon or graphitematerials and prelithiated versions thereof; and combinations thereof.Non-lithiated versions may be used if the cathode side contains lithiumpolysulfides or other lithium sources when the cell is made.

A possible lithium metal cell may be comprised of an anode currentcollector, an electrolyte phase (optionally but preferably supported bya porous separator, such as a porous polyethylene-polypropyleneco-polymer film), a cathode of the instant invention, and an optionalcathode collector. This cathode current collector is optional becausethe presently invented layer of porous graphene structure, if properlydesigned, can act as a current collector or as an extension of a currentcollector.

For a sodium ion-selenium cell or potassium ion-selenium cell, the anodeactive material layer can contain an anode active material selected fromthe group consisting of: (a) Sodium- or potassium-doped silicon (Si),germanium (Ge), tin (Sn), lead (Pb), antimony (Sb), bismuth (Bi), zinc(Zn), aluminum (Al), titanium (Ti), cobalt (Co), nickel (Ni), manganese(Mn), cadmium (Cd), and mixtures thereof; (b) Sodium- orpotassium-containing alloys or intermetallic compounds of Si, Ge, Sn,Pb, Sb, Bi, Zn, Al, Ti, Co, Ni, Mn, Cd, and their mixtures; (c) Sodium-or potassium-containing oxides, carbides, nitrides, sulfides,phosphides, selenides, tellurides, or antimonides of Si, Ge, Sn, Pb, Sb,Bi, Zn, Al, Fe, Ti, Co, Ni, Mn, Cd, and mixtures or composites thereof;(d) Sodium or potassium salts; (e) particles of graphite, hard carbon,soft carbon or carbon particles and pre-sodiated versions thereof(pre-doped or pre-loaded with Na), and combinations thereof.

The following examples are presented primarily for the purpose ofillustrating the best mode practice of the present invention and shouldnot be construed as limiting the scope of the present invention.

Example 1: Mesoporous Soft Carbon as a Supporting and ProtectiveBackbone for Selenium

It may be noted at the outset that “soft carbon” is commonly defined asthe carbonaceous material that is graphitizable. By contrast, a hardcarbon is a carbonaceous material that cannot be graphitized even afterheat treatments at a temperature as high as 2,500-3,200° C. Chemicallyetched or expanded soft carbon was prepared from heat-treating a liquidcrystalline aromatic resin (50/50 mixture of anthracene and pyrene) at200° C. for 1 hour. The resin was ground with a mortar and calcined at900° C. for 2 h in a N₂ atmosphere to prepare the soft carbon. Theresulting soft carbon was mixed with small tablets of KOH (four-foldweight) in an alumina melting pot. Subsequently, the soft carboncontaining KOH was heated at 750° C. for 2 h in N₂. Upon cooling, thealkali-rich residual carbon was washed with hot water until the outletwater reached a pH value of 7. The resulting chemically etched orexpanded soft carbon was dried by heating at 60° C. in a vacuum for 24hours. This material can be used in both the anode and the cathode dueto its high specific surface area and its ability to capture and storelithium atoms on its surfaces. These surfaces (inside pores) were alsofound to be particularly suitable for supporting selenium or metalselenide nanocoating or nanoparticles.

Example 2: Expanded “Activated Carbon” (E-AC) as a Supporting andProtective Porous Backbone for Selenium

Activated carbon (AC, from Ashbury Carbon Co.) was treated with an acidsolution (sulfuric acid, nitric acid, and potassium permanganate at aratio of 4:1:0.05) for 24 hours. Upon completion of the reaction, themixture was poured into deionized water and filtered. The treated AC wasrepeatedly washed in a 5% solution of HCl to remove most of the sulfateions. The sample was then washed repeatedly with deionized water untilthe pH of the filtrate was neutral. The slurry was then dried in avacuum oven pre-set at 70° C. for 24 hours. The dried sample was thenplaced in a tube furnace at 1,050° C. for 2 minutes to obtain expandedAC. This material can be used in both the anode and cathode of a lithiumcell due to its high specific surface area and ability to capture andstore Li atoms on its internal surfaces inside the pores. These surfaceswere also found to be particularly suitable for supporting selenium ormetal selenide nanocoating or nanoparticles in mesopores.

Example 3: Chemically Treated (Expanded) Needle Coke as a Supporting andProtective Porous Backbone for Selenium

Anisotropic needle coke has a fully developed needle-shape texture ofoptical anisotropy. Volatile species of the raw coke was estimated to bearound 5 wt. %. Chemical activation was carried out using KOH in areaction apparatus that consisted of a stainless steel tube and a nickelsample holder. KOH activation was carried out at 800° C. for 2 h underAr flow. The coke/KOH ratio was varied between 1/1 and 1/4. Uponcooling, the alkali-rich coke was washed with hot water until the outletwater reached a pH value of 7. The resulting chemically etched orexpanded coke was dried by heating at 60° C. in a vacuum for 24 hours.The treated coke is highly porous, having a pore size range ofapproximately 1-85 nm.

Example 4: Chemically Treated (Expanded) Petroleum Pitch-Derived HardCarbon as a Supporting and Protective Porous Backbone

A pitch sample (A-500 from Ashland Chemical Co.) was carbonized in atube furnace at 900° C. for 2 hours, followed by further carbonizationat 1,200° C. for 4 hours. KOH activation was carried out at 800° C. for2 h under Ar flow to open up the internal structure of pitch-based hardcarbon particles. The hard carbon-based porous structure was found tohave a pore size range of 3-100 nm (mostly <50 nm) and to beparticularly suitable for supporting and protecting selenium or metalselenide nanocoating or nanoparticles lodged therein.

Example 5: Chemically Activated Mesophase Carbon and Production ofFluorinated Carbon as a Supporting and Protective Porous Backbone

Mesocarbon carbon particles (un-graphitized MCMBs) were supplied fromChina Steel Chemical Co. This material has a density of about 1.8-2.2g/cm³ with a median particle size of about 16 μm. This batch ofmesophase carbon was divided into two samples. One sample was immersedin K₂CO₃ at 900° C. for 1 h to form chemically activated mesocarbon. Thechemically activated mesophase carbons showed a BET specific surfacearea of 1,420 m²/g. This material can be used in both the anode andcathode due to its high specific surface area and ability to capture andstore metal atoms on its surfaces. These surfaces were found to beparticularly suitable for supporting and protecting Se nanocoating orparticles.

Another sample was subjected to a fluorination treatment. The mesophasecarbon particles were mixed with a PVDF binder in a NMP solution andcoated onto an Al foil to form an electrode sheet. This electrode sheetwas used as a working electrode in an electrochemical fluorinationtreatment apparatus consisting of a PTFE beaker, a Pt plate counterelectrode, a Pd wire as a reference electrode, and (C₂H₅)₃N-3HF aselectrolyte. The fluorination procedure was carried out at roomtemperature by potential sweeping from −1.0 V to 1.0 V at a 20 mV/s scanrate. X-ray diffraction data indicate that the inter-graphene spacinghas been increased from 0.337 nm to 0.723 nm.

Example 6: Graphitic Fibrils from Pitch-Based Carbon Fibers for Forminga Porous Backbone

Fifty grams of graphite fibers from Amoco (P-55S) were intercalated witha mixture of sulfuric acid, nitric acid, and potassium permanganate at aweight ratio of 4:1:0.05 (graphite-to-intercalate ratio of 1:3) for 24hours. Upon completion of the intercalation reaction, the mixture waspoured into deionized water and filtered. The sample was then washedwith 5% HCl solution to remove most of the sulfate ions and residualsalt and then repeatedly rinsed with deionized water until the pH of thefiltrate was approximately 5. The dried sample was then exposed to aheat shock treatment at 950° C. for 45 seconds. The sample was thensubmitted to a mechanical shearing treatment in a rotating-bladedissolver/disperser for 10 minutes. The resulting graphitic fibrils wereexamined using SEM and TEM and their length and diameter were measured.Graphitic fibrils, alone or in combination with another particulatecarbon/graphite material, can be packed into a mesoporous structure forsupporting selenium and metal polyselenide.

Example 7: Expanded Multi-Walled Carbon Nanotubes (MWCNTs) as aSupporting and Protective Porous Backbone

Fifty grams of MWCNTs were chemically treated (intercalated and/oroxidized) with a mixture of sulfuric acid, nitric acid, and potassiumpermanganate at a weight ratio of 4:1:0.05 (graphite-to-intercalateratio of 1:3) for 48 hours. Upon completion of the intercalationreaction, the mixture was poured into deionized water and filtered. Thesample was then washed with 5% HCl solution to remove most of thesulfate ions and residual salt and then repeatedly rinsed with deionizedwater until the pH of the filtrate was approximately 5. The dried samplewas then exposed to a heat shock treatment at 950° C. for 45 seconds.Expanded MWCNTs, alone or in combination with another particulatecarbon/graphite material, can be packed into a mesoporous structure forsupporting the selenium material.

Example 8: Conductive Web of Filaments from Electrospun PAA Fibrils as aSupporting Layer for the Anode

Poly (amic acid) (PAA) precursors for spinning were prepared bycopolymerizing of pyromellitic dianhydride (Aldrich) and4,4′-oxydianiline (Aldrich) in a mixed solvent oftetrahydrofurane/methanol (THF/MeOH, 8/2 by weight). The PAA solutionwas spun into fiber web using an electrostatic spinning apparatus. Theapparatus consisted of a 15 kV d.c. power supply equipped with thepositively charged capillary from which the polymer solution wasextruded, and a negatively charged drum for collecting the fibers.Solvent removal and imidization from PAA were performed concurrently bystepwise heat treatments under air flow at 40° C. for 12 h, 100° C. for1 h, 250° C. for 2 h, and 350° C. for 1 h. The thermally cured polyimide(PI) web samples were carbonized at 1,000° C. to obtain carbonizednanofibers with an average fibril diameter of 67 nm. Such a web can beused as a conductive substrate for an anode active material. We observethat the implementation of a network of conductive nanofilaments at theanode of a Li—Se cell can effectively suppress the initiation and growthof lithium dendrites that otherwise could lead to internal shorting.

Example 9: Electrochemical Deposition of Se on Various Webs or PaperStructures (External Electrochemical Deposition) for Li—Se, Na—Se, andK—Se Batteries

The electrochemical deposition may be conducted before the cathodeactive layer is incorporated into an alkali metal-selenium battery cell(Li—Se, Na—Se, or K—Se cell). In this approach, the anode, theelectrolyte, and the integral layer of mesoporous structure (serving asa cathode layer) are positioned in an external container outside of alithium-selenium cell. The needed apparatus is similar to anelectroplating system, which is well-known in the art.

In a typical procedure, a metal polyselenide (M_(x)Se_(y)) is dissolvedin a solvent (e.g. mixture of DOL/DME in a volume ratio from 1:3 to 3:1)to form an electrolyte solution. An amount of a lithium salt may beoptionally added, but this is not required for external electrochemicaldeposition. A wide variety of solvents can be utilized for this purposeand there is no theoretical limit to what type of solvents can be used;any solvent can be used provided that there is some solubility of themetal polyselenide in this desired solvent. A greater solubility wouldmean a larger amount of selenium can be derived from the electrolytesolution.

The electrolyte solution is then poured into a chamber or reactor undera dry and controlled atmosphere condition (e.g. He or Nitrogen gas). Ametal foil can be used as the anode and a layer of the mesoporousstructure as the cathode; both being immersed in the electrolytesolution. This configuration constitutes an electrochemical depositionsystem. The step of electrochemically depositing nanoscaled seleniumparticles or coating in the mesopores is conducted at a current densitypreferably in the range of 1 mA/g to 10 A/g, based on the layer weightof the mesoporous structure.

The chemical reactions that occur in this reactor may be represented bythe following equation: M_(x)Se_(y)→M_(x)Se_(y-z)+zSe (typically z=1-4).Quite surprisingly, the precipitated Se is preferentially nucleated andgrown on massive internal surfaces of mesopores to form nanoscaledcoating or nanoparticles. The coating thickness or particle diameter andthe amount of Se coating/particles may be controlled by the specificsurface area, electrochemical reaction current density, temperature andtime. In general, a lower current density and lower reaction temperaturelead to a more uniform distribution of Se and the reactions are easierto control. A longer reaction time leads to a larger amount of Sedeposited on graphene surfaces and the reaction is ceased when theselenium source is consumed or when a desired amount of Se is deposited.

Example 10: Electrochemical Deposition of Se on Various Mesoporous Websor Paper-Based Cathode Structures in Li—Se, Na—Se, or K—Se Batteries(Internal Electrochemical Deposition)

As an alternative to the external electrochemical deposition, aninternal electrochemical conversion and deposition of Se from anelectrolyte-borne selenium source onto massive internal surfaces ofmesoporous structures was also conducted using a broad array ofmesoporous structures. As a typical procedure, the anode, theelectrolyte, and the integral layer of mesoporous structure are packagedinside a housing to form a lithium-selenium cell. In such aconfiguration, the battery cell itself is an electrochemical depositionsystem for pre-selennization of the cathode and the step ofelectrochemically depositing nanoscaled selenium particles or coating onthe internal surfaces occurs after the lithium-selenium cell isfabricated and conducted during the first charge cycle of the Li—Secell.

As a series of examples, lithium polyselenide (Li_(x)Se_(y))- and sodiumpolyselenide (Na_(x)Se_(y))-containing electrolytes with desired x and yvalues (e.g. x=2, and y=6-10) dissolved in solvent were prepared bychemically reacting stoichiometric amounts of selenium and Li₂Se orNa₂Se in polyselenide free electrolyte of 0.5 M LiTFSI+0.2 M LiNO₃ (or0.5 M NaTFSI+0.2 M NaNO₃) in DOL/DME (1:1, v:v). The electrolyte wasstirred at 75° C. for 3-7 hours and then at room temperature for 48hours. The resulting electrolytes contain different Li_(x)Se_(y) orNa_(x)Se_(y) species (e.g. x=2, and y=6-10, depending upon reactiontimes and temperatures), which are intended for use as a selenium sourcein a battery cell.

In a Li—Se or Na—Se cell, one of these electrolytes was selected tocombine with an anode current collector (Cu foil), an anode layer (e.g.Li metal foil or Na particles), a porous separator, a layer of porousgraphene structure, and a cathode current collector (Al foil) to form aLi—Se or room temperature Na—Se cell. The cell was then subjected to afirst charge procedure using a current density ranging from 5 mA/g to 50A/g. The best current density range was found to be from 50 mA/g to 5A/g.

Examples of the metal polysulfide (M_(x)Se_(y)) materials, solvents,graphene materials, and exfoliated graphite materials used in thepresent study are presented in Table 1 below:

TABLE 1 Selected examples of the metal polyselenide materials, solvents,mesoporous structure materials used in the present study. Selenium Typeof mesoporous structure in source (M_(x)Se_(y)) Solvent Li/Na/K saltsthe cathode Li₂Se₆ DOL/DME LiTFSI CSC, CHC, EAC, CCB Li₂Se₉ DOL/DMELiTFSI CSC, CHC, EAC, CCB Na₂Se₅ Tetra ethylene NaTFSI C-CNT, N-CNT,B-CNT, glycol dimethyl D-CNT, I-CNT ether (TEGDME) Na₂Se₆ TEGDME NaTFSIC-CNT, CCF, CGF, CC-PF K₂Se₆ TEGDME KTFSI C-coke, A-MC, MC, ES-NF,VG-CNF, VG-GNF MgSe₆ Diglyme/ [Mg₂Cl₃][HMDSAlCl₃] M-NW, MC-NW, CP-NW,tetraglyme (HMDS = hexamethyldisilazide) CP-NF MgSe₄ Diglyme/[Mg₂Cl₃][HMDSAlCl₃] CSC, CHC, EAC, CCB tetraglyme (HMDS =hexamethyldisilazide) CuSe₂ NH₄OH or HCl or CuCl₂ C-CNT, N-CNT, B-CNT,H₂SO₄ D-CNT, I-CNT Cu₈Se₅ NH₄OH or HCl or CuCl₂ CSC, CHC, EAC, CCB H₂SO₄ZnSe H₂SO₄ solution ZnSO₄ CSC, CHC, EAC, CCB Al₂Se₃ H₂SO₄ Al₂(SO₄)₃C-CNT, CCF, CGF, CC-PF SnSe₂ HNO₃ and HCl SnCl₂ C-coke, A-MC, MC, ES-NF,VG-CNF, VG-GNF SnSe HCl SnCl₂ C-coke, A-MC, MC, ES-NF, VG-CNF, VG-GNF

Several series of Li metal-Se, Na metal-Se, Na-ion Se, and Li-ion Secells were prepared using the presently prepared cathode. The firstseries is a Li or Na metal cell containing a copper foil as an anodecurrent collector and the second series is also a Li or Na metal cellhaving a nanostructured anode of conductive filaments (based onelectrospun carbon fibers) plus a copper foil current collector. Thethird series is a Li-ion cell having a nanostructured anode ofconductive filaments (based on electrospun carbon fibers coated with athin layer of Si using CVD) plus a copper foil current collector. Thefourth series is a Li-ion cell having a graphite-based anode activematerial as an example of the more conventional anode.

Comparative Examples 10a: Mixing of Selenium with Graphene Sheets orActivated Carbon Particles Via Ball-Milling

Selenium particles and graphene sheets (0% to 49% by weight of Se in theresulting composite) were physically blended and then subjected to ballmilling for 2-24 hours to obtain Se-graphene composite particles(typically in a ball or potato shape). For comparison, graphene sheetsonly (without Se) were also ball-milled to obtain ball- or potato-shapedgraphene particles. The particles, containing various Se contents, werethen made into a layer of graphene structure intended for use in thecathode. Another series of samples for comparison were made undersimilar processing conditions, but with activated carbon particlesreplacing graphene sheets.

Example 11: Some Examples of Electrolytes Used

A wide range of lithium salts can be dissolved in a wide array ofsolvents, individually or in a mixture form. Both ether- andcarbonate-based solvents are suitable for use in an electrolyte for aLi—Se cell. The following are good choices for lithium salts that aredissolved well to a high concentration in selected solvents: lithiumborofluoride (LiBF₄), lithium trifluoro-metasulfonate (LiCF₃SO₃),lithium bis-trifluoromethyl sulfonylimide (LiN(CF₃SO₂)₂ or LITFSI),lithium bis(oxalato)borate (LiBOB), lithium oxalyldifluoroborate(LiBF₂C₂O₄), and lithium bisperfluoroethy-sulfonylimide (LiBETI). Theseselected solvents are DME/DOL mixture, TEGDME/DOL, PEGDME/DOL, andTEGDME. A good electrolyte additive for helping to stabilize Li metal isLiNO₃. Useful sodium salts and potassium salts include sodiumperchlorate (NaClO₄), potassium perchlorate (KClO₄), sodiumhexafluorophosphate (NaPF₆), potassium hexafluorophosphate (KPF₆),sodium borofluoride (NaBF₄), potassium borofluoride (KBF₄), sodiumhexafluoroarsenide, potassium hexafluoroarsenide, sodiumtrifluoro-metasulfonate (NaCF₃SO₃), potassium trifluoro-metasulfonate(KCF₃SO₃), bis-trifluoromethyl sulfonylimide sodium (NaN(CF₃SO₂)₂),sodium trifluoromethanesulfonimide (NaTFSI), and bis-trifluoromethylsulfonylimide potassium (KN(CF₃SO₂)₂). Good solvents are DME/DOLmixture, TEGDME/DOL, PEGDME/DOL, and TEGDME.

Room temperature ionic liquids (RTILs) are of great interest due totheir low volatility and non-flammability. Particularly useful ionicliquid-based electrolyte systems include: lithium bis(trifluoromethanesulfonyl)imide in a N-n-butyl-N-ethylpyrrolidiniumbis(trifluoromethane sulfonyl)imide (LiTFSI in BEPyTFSI),N-methyl-N-propylpiperidinium bis(trifluoromethyl sulfonyl)imide(PP₁₃TFSI) containing LiTFSI, N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide (DEMETFSI) containingLiTFSI,.

Example 12: Evaluation of Electrochemical Performance of Various Li—Se,Na—Se, and K—Se Cells

Charge storage capacities were measured periodically and recorded as afunction of the number of cycles. The specific discharge capacity hereinreferred to is the total charge inserted into the cathode during thedischarge, per unit mass of the composite cathode (counting the weightsof cathode active material, conductive additive or support, binder, andany optional additive combined). The specific charge capacity refers tothe amount of charges per unit mass of the composite cathode. Thespecific energy and specific power values presented in this section arebased on the total cell weight. The morphological or micro-structuralchanges of selected samples after a desired number of repeated chargingand recharging cycles were observed using both transmission electronmicroscopy (TEM) and scanning electron microscopy (SEM).

Shown in FIG. 4 are charge and discharge cycling results of three Li—Secells, one featuring a chemically treated soft carbon (C-SC)-basedcathode containing electrochemically deposited selenium coating of thepresent invention, one containing chemically deposited selenium in C-SC,and the third containing a cathode material prepared by ball-milling amixture of Se powder and activated carbon powder. The dischargecapacities of the above three cells are plotted as a function of thenumber of charge-discharge cycles. The cathode layers in the three Li—Secells were designed to have approximately 65% by weight of Se depositedtherein. Presumably, the resulting composite or hybrid cathode of eachcell should exhibit a maximum specific capacity of 675×65%=439 mAh/g.However, the cathode layer prepared by chemical deposition of Seexhibits a specific capacity of only 340 mAh/g, which means a Seutilization efficiency of 340/439=77.4%. The cathode layer prepared byball milling of AC-Se powder mixture exhibits a specific capacity ofonly 301 mAh/g, which means a Se utilization efficiency of 301/439=68%.In contrast, the cathode layer having electrochemically deposited Secoating (thickness 6.5 nm) prepared according to an embodiment of theinstant invention delivers an Se utilization efficiency of 391/439=89%.This dramatic difference in efficiency is truly stunning. Data from manymore samples investigated are summarized in Table 2 below:

TABLE 2 Selenium utilization efficiency data for alkali metal-seleniumcell cathodes containing various Se contents, Se coating thicknesses orparticle diameters, substrate materials, and Se deposition methods.Cathode Discharge discharge capacity, Sample Cathode active % of Se andthickness capacity mAh/g, based Se utilization ID layer material ordiameter (nm) (mAh/g) on Se weight efficiency CSC-1 CSC 95% Se; 6.1 nm575 605 89.67% CSC-2 CSC 95% Se; 3.4 nm 584 615 91.07% CSC-3 CSC 75% Se;7.1 nm 423 564 83.56% CSC-c1 CSC 75% Se (solution) 365 487 72.10% CSC-c2CSC 75% Se + CSC; ball- 320 427 63.21% milled CSC-c3 SC, non-treated 75%Se; external 388 517 76.64% CHC-1 CHC 70% Se, External 440 629 93.12%CHC-c1 CHC 70% Se, Chemical 354 506 74.92% reaction EAC-1 EAC 70% Se,External 435 621 92.06% EAC-c1 EAC 70% Se, solution 348 497 73.65%C-CNT1 C-CNT 70% Se, External 430 614 91.01% C-CNT2 C-CNT 70% Se, in acell 418 597 88.47% C-CNT-c1 C-CNT 70% Se; solution 337 481 71.32%deposition C-CNT-c3 CNT, non-treated 70% Se; External 352 503 74.50%A-MC1 A-MC 85% Se; 9.2 nm, in a 520 612 90.63% cell A-MC-2 A-MC 85% Se;19.2 nm, 523 615 91.15% external A-MC-c1 A-MC 85% Se; Chemical 475 55982.79% reaction A-MC-c2 MC, non-treated 85% Se: external 519 611 90.46%C-coke1 C-coke 85% Se; external 526 619 91.68% C-coke2 C-coke 85% Se;solution 423 498 73.73%

The following observations can be made from the data of Table 2:

-   -   1) Thinner coatings prepared according to the instant invention        lead to higher efficiency of Se utilization given comparable Se        proportion. Given comparable Se coating thickness, the Se        utilization efficiency is relatively independent of the Se        proportion deposited in the mesopores.    -   2) The presently invented electrochemical deposition method is        significantly more effective than all conventional methods        (solution deposition, ball-milling, chemical reaction-based        deposition, etc.) in terms of imparting Se utilization        efficiency to the resulting cathode structure of a Li—Se, Na—Se,        or K—Se cell.    -   3) Both external electrochemical deposition and internal        electrochemical deposition are capable of depositing a high Se        proportion while maintaining a thin Se coating (hence, high Se        utilization efficiency). Prior art methods are not capable of        achieving both.

The data shown in FIG. 4 also indicate that the presently invented Li—Secell does not exhibit any significant decay (only 8.7%) even after 550cycles. In contrast, the prior art cell containing ball-milled Se/ACcathode suffers a 43.5% capacity decay after 550 cycles. In fact, itsuffers a 20% capacity decay after 220 cycles. The cycle life of alithium battery cell is usually defined as the number of cycles when thecell reaches a 20% capacity decay. These results are quite unexpectedconsidering that the same type of chemically treated SC was used as thesupporting material and the same amount of selenium was used in thesethree cell cathodes.

FIG. 5 shows Ragone plots (cell power density vs. cell energy density)of two Li—Se cells. The presently invented Li—Se cell featuring achemically treated/expanded needle coke (C-NC)-based cathode containingelectrochemically deposited selenium particles (83% by weight of Se)exhibits an exceptional cell energy density (as high as 389 Wh/kg, basedon total cell weight). The same cell also delivers a maximum powerdensity as high as 2768 W/kg, which is significantly higher than typicalpower densities (up to 500 W/kg) of lithium-ion batteries.

In contrast, the C-NC-based cathode containing chemically depositedselenium particles (64% Se) enables the Li—Se cell to store up to 201Wh/kg and delivers a maximum power density of 1,123 W/kg. These aresignificantly lower than those of the presently invented cell. Thesedata have clearly demonstrated the unexpected yet superior effectivenessof the presently invented external and internal electrochemicaldeposition methods and the chemical treatment or expansion approach.

Shown in FIG. 6 are Ragone plots (cell power density vs. cell energydensity) for 4 alkali metal-selenium cells. The first cell is a Na—Secell featuring a chemically treated mesocarbon (C-MC)-based cathodecontaining electrochemically deposited selenium particles (70% Se),which exhibits the highest energy density and power density among thefour cells. The second is a Na—Se cell featuring a C-MC-based cathodecontaining chemically deposited selenium particles (70% Se). Clearly,the cathode having chemically deposited Se is not as effective as thepresently invented cathode of electrochemically deposited Se inproviding high energy density and power density. The third cell is aK—Se cell featuring a C-CMC-based cathode containing electrochemicallydeposited selenium particles (70% Se), and the fourth cell is a K—Secell featuring a C-CMC-based cathode containing solution-depositedselenium particles (70% Se). Again, the presently inventedelectrochemical method is so much superior. The data in FIG. 6 alsoindicate that the presently invented Na—Se cells can store an energydensity up to 201 Wh/kg, which is higher than those of Li-ion batteries.Additionally, even K—Se cells can store up to 142 Wh/kg, better thanmost of the Li-ion cells. These highly surprising results are a goodtestament to the effectiveness of the presently invented method ofdepositing selenium in the pores of a mesoporous structure.

In summary, the present invention provides an innovative, versatile, andsurprisingly effective platform materials technology that enables thedesign and manufacture of superior alkali metal-selenium rechargeablebatteries. The alkali metal-selenium cell featuring a cathode containinga mesoporous structure having pores of 0.5-50 nm with ultra-thinselenium electrochemically deposited therein exhibits a high cathodeactive material utilization rate, high specific capacity, high specificenergy, high power density, little or no shuttling effect, and longcycle life. When a similarly configured anode structure (with noselenium) or a nanostructured carbon filament web is implemented at theanode to support a lithium film (e.g. foil), the lithium dendrite issueis also suppressed or eliminated.

The invention claimed is:
 1. An electrochemical method of producing apre-selenized active cathode layer for a rechargeable alkalimetal-selenium cell, said method comprising: (a) preparing an integrallayer of a mesoporous structure of a carbon, graphite, metal, orconductive polymer, wherein said mesoporous structure has mesoscaledpores of 0.5-50 nm and a specific surface area greater than 100 m²/g andwherein said carbon, graphite, metal, or conductive polymer is selectedfrom chemically etched or expanded soft carbon, chemically etched orexpanded hard carbon, exfoliated activated carbon, chemically etched orexpanded carbon black, chemically etched multi-walled carbon nanotube,nitrogen-doped carbon nanotube, boron-doped carbon nanotube, chemicallydoped carbon nanotube, ion-implanted carbon nanotube, chemically treatedmulti-walled carbon nanotube with an inter-planar separation no lessthan 0.5 nm, chemically expanded carbon nanofiber, chemically activatedcarbon nanotube, chemically treated carbon fiber, chemically activatedgraphite fiber, chemically activated carbonized polymer fiber,chemically treated coke, activated mesophase carbon, mesoporous carbon,electrospun conductive nanofiber, highly separated vapor-grown carbon orgraphite nanofiber, highly separated carbon nanotube, carbon nanowire,metal nanowire, metal-coated nanowire or nanofiber, conductivepolymer-coated nanowire or nanofiber, or a combination thereof; (b)preparing an electrolyte comprising a non-aqueous solvent and a seleniumsource dissolved or dispersed in said solvent; (c) preparing an anode;and (d) bringing said integral layer of mesoporous structure and saidanode in ionic contact with said electrolyte and imposing an electriccurrent between said anode and said integral layer of mesoporousstructure, serving as a cathode, with a sufficient current density for asufficient period of time to electrochemically deposit nanoscaledselenium particles or coating directly on said graphene surfaces to formsaid pre-selenized active cathode layer, wherein said particles orcoating have a thickness or diameter smaller than 20 nm.
 2. The methodof claim 1, wherein said selenium source is selected from M_(x)Se_(y),wherein x is an integer from 1 to 3 and y is an integer from 1 to 10,and M is a metal element selected from an alkali metal, an alkalinemetal selected from Mg or Ca, a transition metal, a metal from groups 13to 17 of the periodic table, or a combination thereof.
 3. The method ofclaim 2, wherein said metal element M is selected from Li, Na, K, Mg,Zn, Cu, Ti, Ni, Co, Fe, or Al.
 4. The method of claim 2, wherein saidM_(x)Se_(y) is selected from Li₂Se₆, Li₂Se₇, Li₂Se₈, Li₂Se₉, Li₂Se₁₀,Na₂Se₆, Na₂Se₇, Na₂Se₈, Na₂Se₉, Na₂Se₁₀, K₂Se₆, K₂Se₇, K₂Se₈, K₂Se₉,K₂Se₁₀, or a combination thereof.
 5. The method of claim 1, wherein saidanode comprises an anode active material selected from an alkali metal,an alkaline metal, a transition metal, a metal from groups 13 to 17 ofthe periodic table, or a combination thereof.
 6. The method of claim 1,further comprising a procedure of depositing an element Z to said porousgraphene structure wherein said element Z is mixed with selenium orformed as discrete Z coating or particles having a dimension less than100 nm and said Z element is selected from Sn, Sb, Bi, S, Te, or acombination thereof and the weight of element Z is less than the weightof selenium.
 7. The method of claim 6, wherein said procedure ofdepositing element Z includes electrochemical deposition, chemicaldeposition, or solution deposition.
 8. The method of claim 1, whereinsaid nanoscaled selenium particles or coating occupy a weight fractionof at least 70% based on the total weights of said selenium particles orcoating and said carbon, graphite, metal or polymer material combined.9. The method of claim 1, wherein said electrolyte further comprises ametal salt selected from lithium perchlorate (LiClO₄), lithiumhexafluorophosphate (LiPF₆), lithium borofluoride (LiBF₄), lithiumhexafluoroarsenide (LiAsF₆), lithium trifluoro-metasulfonate (LiCF₃SO₃),bis-trifluoromethyl sulfonylimide lithium (LiN(CF₃SO₂)₂), lithiumbis(oxalato)borate (LiBOB), lithium oxalyldifluoroborate (LiBF₂C₂O₄),lithium oxalyldifluoroborate (LiBF₂C₂O₄), lithium nitrate (LiNO₃),lithium-fluoroalkyl-phosphates (LiPF₃(CF₂CF₃)₃), lithiumbisperfluoro-ethysulfonylimide (LiBETI), lithiumbis(trifluoromethanesulphonyl)imide, lithium bis(fluorosulphonyl)imide,lithium trifluoromethanesulfonimide (LiTFSI), an ionic liquid-basedlithium salt, sodium perchlorate (NaClO₄), potassium perchlorate(KClO₄), sodium hexafluorophosphate (NaPF₆), potassiumhexafluorophosphate (KPF₆), sodium borofluoride (NaBF₄), potassiumborofluoride (KBF₄), sodium hexafluoroarsenide, potassiumhexafluoroarsenide, sodium trifluoro-metasulfonate (NaCF₃SO₃), potassiumtrifluoro-metasulfonate (KCF₃SO₃), bis-trifluoromethyl sulfonylimidesodium (NaN(CF₃SO₂)₂), sodium trifluoromethanesulfonimide (NaTFSI),bis-trifluoromethyl sulfonylimide potassium (KN(CF₃SO₂)₂), or acombination thereof.
 10. The method of claim 1, wherein said solvent isselected from 1,3-dioxolane (DOL), 1,2-dimethoxyethane (DME),tetraethylene glycol dimethylether (TEGDME), poly(ethylene glycol)dimethyl ether (PEGDME), diethylene glycol dibutyl ether (DEGDBE),2-ethoxyethyl ether (EEE), sulfone, sulfolane, ethylene carbonate (EC),dimethyl carbonate (DMC), methylethyl carbonate (MEC), diethyl carbonate(DEC), ethyl propionate, methyl propionate, propylene carbonate (PC),γ-butyrolactone (γ-BL), acetonitrile (AN), ethyl acetate (EA), propylformate (PF), methyl formate (MF), toluene, xylene, methyl acetate (MA),fluoroethylene carbonate (FEC), vinylene carbonate (VC), allyl ethylcarbonate (AEC), a hydrofluoroether, a room temperature ionic liquidsolvent, or a combination thereof.
 11. The method of claim 1, whereinsaid anode, said electrolyte, and said integral layer of mesoporousstructure are disposed in an external container outside of alithium-selenium cell and said step of electrochemically depositingnanoscaled selenium particles or coating in said mesoscaled pores isconducted outside said lithium-selenium cell and said method furtherincludes a step of incorporating said pre-selenized active cathode layerin said lithium-selenium cell.
 12. The method of claim 1, wherein saidanode, said electrolyte, and said integral layer of mesoporous structureare disposed inside a lithium-selenium cell and said step ofelectrochemically depositing nanoscaled selenium particles or coating insaid mesoscaled pores is conducted after said lithium-selenium cell isproduced.
 13. The method of claim 1, wherein said anode, saidelectrolyte, and said integral layer of mesoporous structure are part ofa lithium-selenium cell and said step of electrochemically depositingnanoscaled selenium particles or coating in said mesoscaled pores occursafter said lithium-selenium cell is fabricated and is conducted during afirst charge cycle of said cell.
 14. The method of claim 1, wherein saidnanoscaled selenium particles or coating occupy a weight fraction of atleast 80%.
 15. The method of claim 1, wherein said nanoscaled seleniumparticles or coating occupy a weight fraction of at least 90%.
 16. Themethod of claim 1, wherein said nanoscaled selenium particles or coatinghave a thickness or diameter smaller than 10 nm.
 17. The method of claim1, wherein said nanoscaled selenium particles or coating have athickness or diameter smaller than 5 nm.
 18. The method of claim 1,wherein said nanoscaled selenium particles or coating have a thicknessor diameter smaller than 3 nm.
 19. The method of claim 1, wherein saidmethod is conducted in an electrochemical chamber that is outside of anintended alkali metal-selenium cell and said method further contains astep of combining said pre-selenized active cathode layer, an alkalimetal anode layer, and an electrolyte to form said alkali metal-seleniumcell.
 20. The method of claim 1, wherein said method is conducted insidean intended alkali metal-selenium cell and during the first charge ordischarge cycle of the cell.